Cards don't make good presentation pieces, but if that's what he's got - and they're commons - you can duct tape them in all honesty. In my adventures with my nephew (who was looking to do the same thing), I got him to look at 8"x10" photos instead. Much more presentable and really cost very little since they're reproductions.
For about $3 a picture, I bought some Mylar covers to put on top. I then pinned them to a corkboard I put on his bedroom wall. Four years later, he's still looking for more pictures - where I take him to a show here and there - and wants to build on it. So far he has about 12 and it looks really sweet.
My friend had a card shop in my neihborhood in the 90s when cards were selling well. He had a nice wall-mount thing that was the shape of a ballpark with slots at every position. The idea was that you'd fill those positions with a card. Looked nice but I never saw one cheaper than $30 and some being hand-made over $200.
Personally, I'd go black foamcore since the white stock of the cards will at least stand out. For mounting - go cheap IMO. These cards can be replaced for pennies or your son simply gets next year's as a replacement.
"Chuckie doesn't take on 2-0. Chuckie's hackin'." - Chuck Carr two days prior to being released by the Milwaukee Brewers