Don't get a chair. You have to clean them and they get messy and nasty. Get the padded seat that sits on the big potty, and a stool for when he's old enough to go alone. That way he won't ever have issues with it. You'll have to take him at this age anyway, so it's no big deal. My daughter was in panties by 19 months, but I had to remind her and take her.
Be sure to get one with the little splash guard. They are a must for little boys, but little girls can be helped by them too!
Let him put the seat on the potty, you help him up, and let him flush. That's so much fun at that age!