D.B.
I think it's nice that you want to include him, and I think a gift that's not too personal would make the most sense for the first year you're doing this.
A gas card is nice, same with movie tickets or a movie/pizza thing. Yes, she can enjoy it too, but that's okay. It sends the message that you are supportive of their relationship.
Also, iTunes gift cards are usually appreciated.
If he's into something like running, a gift certificate to a running store would be fine. My son always needs new running socks or those wicking shirts. You can avoid buying the actual item by doing a gift certificate, but if you want to give him a "thing" to open, then running socks or even some reflective gear for night running is more impersonal than, say, Under Armour!