A 135/2.8 was my first lens. Very cheap, very useful. (In fact I got one for my Nikons a few weeks ago and finally put some film behind it a couple of weeks ago - and the shots are beautiful.) The 135/2.5 Canon equivalent would be a fine substitute.
If Canon has a 100/105 mm lens that's also great. Nikon's 105/2.5 is a beautiful lens. Some above have mentioned the 100/2.8 and it would be just fine.
I'd stay away from the zooms for now. They are more flexible but harder to learn with, in my opinion. If your son can learn to use a 50 and 100/135 effectively, he'll be a much better user of a zoom later.