Depends. A lot of people have major growth spurts before they are 18 and then yes, they do stop after 18/19 besides maybe half an inch. If you did not have a major growth spurt, a lot of people still can grow until they are into their mid-20's. I was no more than 5'9" or so when I was 18, and I am now close to 5'11".
His prematurity may have stunted his growth, but both of his brothers are over 6'0". I would say even with that, he could grow at least a few inches. If he gets to 5'11" and then just bulks up, that's sufficiently large... Especially since he plays a far more skilled game than Jason anyways.