Putting rear hand brakes onto a 16" Diamondback Jr Viper
It looks like to 20" version of the bike comes with a rear linear pull brake, but the 16" has a coaster brake. This is common practice with kids bikes as children young enough to be on a 16" bike don't have enough hand strength to operate a brake lever effectively.

I looked into doing the same thing when my daughter had a 16" bike, but found that it is not workable. In general there is no place to mount a brake caliper on a 16" bike. There is often a seat stay bridge with a hole in it but it is intended to mount a fender. The hole is too far away from the rim to mount a caliper.

My advice would be to save your money for the next bike you get your son when he grows out of the 16".