A confused dad :(

I often think it would be easier to negotiate with terrorists !

It is normally cheaper to buy a new bike than buy the bits to build your own. Buying a good quality bike second hand will also save money as will checking out the sales of 2013 bikes currently on at the moment.Do you have a budget in mind ?

If your son is still growing a fair bit this may end up being more of a stop gap bike until he is fully grown but it is good he is interested in mountain biking as he will get fit and learn mechanical skills. You could suggest he gets a full bike and then spends some time stripping it down and upgrading it a bit at a time. This is how I learnt and it is cheaper / less risky. You can get fairly cheap bike tool sets which will do most jobs and park tools have various video's you can watch about installing, tweaking and replacing compenents.

See what you think and people on here can suggest suitable bikes etc for your budget and needs

The important factors that are going to affect what parts fit your son's new frame are:

1. Dropouts (the two holes where the rear axle mounts into the frame).
2. Bottom bracket shell (the bit where the pedal cranks pass through the frame) You will need to source a bottom bracket to fit the shell size and thread (if any), as well as the cranks you want to fit.
3. Headtube (the tube at the very front of the frame, that houses the headset (bearings that hold the fork in place)) - you'll need to source a headset that fits the headtube and the forks that you choose.

For each of the above, there are several current "standards" that different frames use depending on their intended use, manufacturer and year of manufacture.

The best thing you can do is get your son to find out what year Hoodoo it is and the kindly folks here should be able to tell you what standard each of the above is. Then you can work out what parts (if any) you have that can be used, and what you need to find.

Best bet is to buy a local bike that is cheap with a beaten up frame, swap over that which suites (do a little homework first) and then top up the rest from the classifieds and ebay/gumtree.

I did this with my bike, link in my sig, first iteration it was on cable discs, cheap wheels and forks, but rode pretty well, that cost about £250 to get rolling (inc the frame at about £35).

What is he currently riding? Just because it came from ebay doesn't mean it isn't a proper MTB, and even if it isn't many of the parts may be transferable till you can upgrade them. (Bars, stem, seat, seatpost, possibly bottom bracket and cranks). I'm guessing it isn't disc brakes in which case the wheels won't transfer. Nothing wrong with rim brakes - but there's nothing wrong with cocoa and a cardie - doesn't mean a 15 year old will like them..

Pretty sure the hoodoo will take a 1 1/8inch fork steerer which are pretty common as till about 2 years ago that was pretty much the only standard. It will also probably be the biggest single expense on this build - and the thing that will affect proper MTBing the most. Anything with that steerer size from rockshox, fox or marzocchi will prob be ok for the job. Obviously they have high and low end stuff with priced to match. Check what the max travel fork you can fit is - i would guess 120mm. i would imagine a 15 yr old will be wanting to fit 200mm travel monsters on it! Longer isnt nevessarily better - it is for a different job. 100mm travel is great for general XC riding. 120-130mm for stuff a bit bigger - makes uphill harder and downhill easier as well as weighing more. Generally suntour forks found on cheaper bikes are poor (and hence in abundance on ebay after upgrading)

Bars come in 2 sizes, although 31.8mm (oversize) are probably the most common, so your stem needs to match the bars and the steerer tube on The fork (although you can get a shim you are better buying right in the first place).

Front derailleurs come with different size bands depending on the seat tube diameter (and other methods of attachment too) and are top pull or bottom pull depending where the cable routes. Rear derailleurs are pretty straightforward, SRAM and shimano are both fine for gears - if you are a novice best advice is not to mix and match though, use one or the other throughout for shifters and derailleurs, different models from the same brand should be fine. Ie SRAM x7 shifters with an x9 derailleur

Seat posts and collars (if not on the frame) come in a variety of sizes which need to match the frame, saddles, grips are pretty universal.

Hydraulic disc brakes fill many with dread - but aren't that scary. There are 2 fitting standards - post and IS. Adapters from one to another cost peanuts so don't worry too much about that. The fluid they run on differs with brands (hydraulic fluid or mineral oil)

Bottom brackets come in 2 sizes and many different types - which need to match the cranks. Wheels come in more sizes now but 26" will be by far the most common, which is what you need. SRAM and shimano are completely interchangeable for cassettes and freewheels.

Depending what is on the frame, you will need a headset press (and remover if a damaged one is there). A tool to fit the bottom bracket (different depending on the type). If everything you buy is working that should see you through - other wise cone spanners for wheels, chain whip, cassette tool, bleed kit specific to brakes spring immediately to mind.

I would be amazed if you build that piecemeal for less than the bike cost new! It was the best budget buy at the time (for quite a while) in one of the mags, so worth building. But I would highly recommend scavenging the parts from another bike to build it.

All of the above is sweeping generalisation on the whole (you could fill a whole book on the subject). Expensive as it may be don't forget to factor the quality time into the cost - your lad actually has a point there. And if he continues with his MTBing he will have learnt a lot of valuable maintenance skills that will save him a ton of cash in the future, as well as potential hardship while stranded with a mechanical in the middle of nowhere that is actually easy to fix...