What should I do about my lazy down syndrome adult son?

He took tap lessons til he was twelve he was good at it but quit to take up drawing and he was good at that but quit when he was about 18 a couple years after his older brother committed suicide.  Now he's 30 and the only thing he does is play video games, shop, and smoke weed.  He gets money from disability and doesn't seem to intend to get a job.  I talked him into taking a cake decorating course and we took courses one and three together when he was 19.  He's 30 now shouldn't he do something with his life even though he has down syndrome and is intellectually disabled.  I think he's good at art stuff but when I try to get him into it all he says is no that's old and he says he doesn't want to be famous.  He doesn't have to be famous but he should do something with his life.  I have ideas of how to get through to him but it doesn't turn out the way I expected.  Smoking weed kills brain cells and he has it bad enough already.  He takes therapy but says he will never tell he smokes should I get involved with the therapy.  I think therapy can help if they know the truth.