Can pubescent boys be given dihydrotestosterone to ensure they get a bigger penis?

Yes they can. It is too late if you wait until after puberty. We use 50 mg of Depo-Testosterone by intramuscular injection monthly for 2 - 4 injections depending on response. Hormone is given in a relatively small dose, if he is a big kid, also widely spaced to avoid stunting his growth by so called epiphyseal fusion of his bones.