S.H.
He will do it when he is ready.
My son was speech delayed, the speech therapist said SOMETIMES they may be delayed in pottying or other things, etc. My son, started... at about 3 years old.
We just kept a potty chair out and about in whatever room he was in. Didn't make a fuss about it. And he'd be naked on the bottom. Then one day... HE sat on it. Nothing came out but he was proud. Then one day he sat on it and a dribble came out. He was proud. Then one day he sat on it and actually peed. He was proud. We complimented him. Then gradually he did it more often... on his own.
He'd even scream "ITS' MY BODY!" but he learned!