Patches don't stretch with the knitting, but knitting does stretch. I'm not sure where you're going to place them, but keep that in mind. I'd think one or two on the edge or corners might be okay.
I probably wouldn't even attempt it, but if I was going to I'd probably hand sew. :shrug: Personally I think I'd just do it in the colors of whatever he's in. Like army green stripes or something.