I agree with little keepsakes, something JUST FOR HIM, if he is having his own room maybe a nice name plaque?
I found that even though we had sheets etc for DD1 i really wanted a blanket just for DD2, i wanted to be able to keep 1 or 2 of their blankets for them for later on.
A towel or blanket with his name on it?
A 'brotherly love' photo frame (or 2, 1 each)? Everywhere was sold out of sisterly love ones when i had DD2 and i would have really liked a frame each for the girls with a picture of their first meeting... i am still on the lookout but might make something myself.

I know it's not a given but i think sometimes it's nice to get the other child a little something aswell, maybe a little toy car... depending on the older childs age they can get a bit funny about all the attention on mum and presents for bub so even the phot frame works well.