Looks sweet man, I used to ride park BMX myself for a while, but I prefer bike trials because of the nature. It requires more balance and care - park BMX only really needs momentum for the ramps...
Anyways, if he is such a keen rider of BMX then 24" would probably be best. You can do traditional trials stuff on them, although they do tend to lumber a little unlike the smaller mod bikes. But like I said, the geo is much closer to a BMX than other trials bikes, so he may get on better with one. There are a few Onza Zoots in the for sale section, and some of these can go for as little as £250 - £300 (new they're only about £500 or so)