there's no way he would fit a 6.5 if he wears clarks in 9.5.
I have clarks db in 7 which are a little big and where a 6.5 in chucks. Shoes like nike sb's with all that padding I wear a 7.5-8 and nike sb janoski's I were 7.5.
I'd say size down 1 from your regular shoe size. If you go to converse site they have this thing where you choose your reg shoe size and then it tells you what size in converse you are. But like mentioned it's best to go to a store. FootLocker, Journey's, Actionfoot or what ever it's called, nordstrom..they all carry them.