Another vote for the Leapster! My son got his first one at 3, it has helped with with small motor skills, abc, counting, sequencing etc. The games for your sons age are all learning games...so why wouldn't you want them to have fun and learn at the same time!
I was considering this past christmas to move him to a DSI (he is 6 now) as his older cousins have them and a couple of friends too. However the DSI is more games, less learning. Also, I think the DSI is pricey. My son drops his Leapster and misplaces the games. ( i have everything in an old camera bag to put everything into). The DSI games are tiny! and they are expensive. I'm sure he would lose them.
So I gave him the Star Wars version of the Leapster (its black) and looks more "big boy" looking. He also got Star Wars Reading and Math games.
This version of the Leapster you can plug into your computer and get more things, however we haven't done that. He is content just playing the games on the Leapster.
I would suggest buying a new one, as our son's first Leapster is a little quirky, sometimes it won't play a game. It is resolved by cleaning the contact with rubbing alcohol. Read the reviews on Amazon. It seems it effected some of the older models. You can get a basic one for around $35 and the games from $15 - $25.
It is the go to toy for all travel, plane, car etc. Also, when I need him to be distracted, like going to the dentist to get teeth cleaned, grocery shopping. I never feel guilty about it because I know he is learning along the way. Have a great trip!