I was going to say a wallet, but Phlorence_is_back did. Does he like to fish? Get him a new fishing pole, or a tackle box. Get him a pocket knife or a t-shirt. Get him a coffee mug or a hat. There are endless possibilities. If you told us what his likes and interests are I could narrow it down.
Get him a subscription to a good magazine like Scientific American, Popular Science, Discover, Smithsonian, something that he will like to read. It can be something to do with his work but I would make it something he likes for a hobby or maybe something totally different to take his mind off his work. Maybe ask your mom for help on what it should be. You can go to a store that sells magazines and look at them, buy one you think he would like and use the card in it to get the subscription. Ask mom about how to pay for it.