Surely it doesn’t have v-brake mounts, so you’re limited to calipers and compatible levers (don’t know if that means mini-v levers will be right or wrong?), and even then maybe only the front brake unless you can drill a hole for the back brake? The forks on our balance bike have a hole for a caliper, think that is normal.
Oh, and take him to a big bmx track at a quiet time, good place to learn to go down hills with a decent smooth run out and no need for a brake.
Oh, and the otherwise absolutely brilliant strider balance bike has a stupid push down brake rather than handlebars, which they claim is better for little kids. Having seen it tested in somewhere with proper steep hills, and going onto a proper bike at 3 and a bit, I can safely say that I disagree with them about the brake, they should just bung a normal brake on there, way better even for a little kid than the silly foot brake.