Hi, M.,
I recommend staying away from the foam ones -- they always come apart. Glue never sticks to them for long. Go with a pre-made wood one that he can paint and decorate.
Or just use the craft sticks. That's the only frame craft I still have - my son made it when he was three (he's 10 now) and it is still hanging on my fridge with a picture of the whole family in it. I had all the kiddos (I ran a home daycare and the theme that week was family) paint four craft sticks any way they wanted. We let them dry, then glued them together in a square, and taped a photo to it (from behind, of course!) then put a magnet on it and voila! They really came out cute.
Have fun, with whatever you go with! J.