many ways to do this. I made a rubic's cube in the early 80's for a blind friend...coarse and fine sandpaper, felt, foam, velvet, glitter paper...it doesn't have to be pretty to a sighted friend, but a lot of things you do already will work, i.e. drawing lines with glitter pens, ribbon borders, outlining, ...look at the next card you make, and think of what could work for the child...I bet each card you make has at least one item, texture, ribbon, line, whatever.
Like I said, it doesn't have to be pretty to us, but to a child...think feel. labrinth. puzzle. squares. geometric shapes...even simple shapes, like house, cake, car...almost things you can die-cut.
Hope that helps...and I am going to go through my stuff, and see what I can make.
BTW, do you know if he reads Braille yet, as in just his name?