K.S.
First, put him in underwear. Make a big deal, let him pick his own, etc.
Start with 30 to 45 minutes - set a timer. When the timer goes off, ask, then put him on for no longer than 2 minutes. If he goes, great. If not, reset the timer and go again. As he gets better, start extending the time by 15 minute increments. Eventually, he'll understand that this is where he is expected to go and will start asking as he understands the sensation of having to go.
Also, if you are worried about accidents, like you are going somewhere or he is going to take a nap, put a pull up on OVER the underwear so that if he has an accident he feels it and is uncomfortable. Without the underwear, daipers and pulls ups are so absorbant that they don't really FEEL it.
I used a great book called The Potty Trainer that you download on the internet. Easy read and great tips! My daughter trained at 21 months and my son with Down syndrome at just before 4 years old.
Good luck,
K.