have you tried eating a lot of oatmeal? that always helps my supply. also there is an herbal product called Mothers Milk Plus that can give you a boost.
honestly though it's about more than the milk. my (then 3 year old) DD nursed through my pregnancy, even though i mostly dried up. they like the sucking. think about how many children you know or have heard about, who still use a pacifier or like to drink from a bottle at the age of 3+. they like the sucking.
if you can stand it, i'd say, just keep nursing him and don't worry about the supply so much. that said, i believe in giving limits to a preschool age child. my DD, who is now 4, gets to nurse before bed, and usually on a couple other occasions during the day. but *not always* like her 9 month old brother does. she got to do it constantly too when she was a baby... she knows that she's actually pretty lucky to still be allowed to nurse, every other kid she knows has a mom who said "no more" and i think it helps her appreciate it and deal with the limits.