My older son was totally obsessed with Thomas from the time he was about 20 months until age 6 or 7. It was ALL he played with. Even at age 10 he is still very attached to all his Thomas stuff even if he doesn't play with it. (DS#2 now plays with it.) At one point we had every engine out and more wooden track than any ten kids needed. We started out with the figure eight track and Thomas, Annie, and Clarabel and then just kept adding from there. Personally I would start there. A simple circle or figure eight of wooden track and Thomas. Most ToysRUs seem to have train tables set up you could take him to play with and see what he likes. Some book stores too.
You can get some deals on Ebay but you can also pay more than retail for trains on there, so definetly know the prices. It has been a few years, but I used to get some good deals on
THE TRAIN CELLAR