Road/ CX bike for a teen

After riding drop bar bike for the first time my son has decided that he wants CX bike rather than a hybrid. He is 4ft 11inch with 27 inch inseam. His seat on his bike is at 32inch( if thats any help ) and could say that he needs it up a bit more.
We tried him on Wiggins Rouen Adv 26 ( for me it looked small ) and he was on Frog 70 track bike which he fit I think but it looks to be on a smaller side for him though according to frog website he should fit it. I Don want to get a bike which he would use for a year and we would have to look for another one all over again.
We are looking at secondhand market and don't want to spend more than £250 max £300.