I second the idea of games and puzzles! Will he wear any other dress up clothes besides firefighter...maybe you could suggest that? Also maybe some Little People toys? They have pretty fun cars, trucks, firetrucks, etc. My 3 year old daughter plays with Little People and likes it! Does he watch any TV or movies? My daughter also likes the DVDs of her favorite TV shows.
For the younger one...that is tougher since he has all the hand me downs. Do you have any push toys or anything...I know he can't use it yet, but maybe something to grow into. Also maybe look at the clothes and see specifically if he needs anything that might not fit or be in season from your son and give out more specific requests like mittens, hats, whatever. I suppose you could always ask for diapers!?! Maybe it's a little tacky, I guess, but shoot, I don't know! ;)
Also, just a side note, Gymboree has a ton of super cute fireman clothes on sale right now and they are WAY cute! They are in store and on line...check them out b/c then your son can have the best of both worlds with looking like a fireman in "regular" clothes!