Congrats on your boy ! I have two and they can be a handful, much different than girls, but lots of fun. I'll probably soon be changing their room from nursery animals to a sports theme. We already have an NFL & Indiana Pacers poster hung over the Giraffe. It's so cute - I'll be sad to change it.
I did a light blue faux finish to somewhat resemble clouds on all 4 walls then drew & painted huge animals on three of the walls. I'd suggest maybe blue walls - that can always stay. Then add your browns & reds with accents. They do have sports themed stickie designs for not too much money that are easily removed. Check out Lowes, HD or even Wal Mart, paint stores...
You could also do a stripe below the chair rail in brown & red if you wanted to. Or have a sports themed mural painted on 1 wall.
Babies -R Us may even have a wall paper border to match your bedding, borders aren't usually that difficult to change. I'd hang it low for baby to enjoy as he grows.
Also blue & brown is a pretty & popular combination. Maybe paint 1 wall brown, then use red as accents with pillows, rugs,...