What is a boy before he grows pubic hair?

He's a boy! Nothing more, nothing less. A boy before he grows "pubic" hair is just a young male that has not gone through puberty yet.

Puberty:

The changes of a persons body in which they begin to become and adult capable of reproduction. Puberty is triggered by hormone signals from the brain. In boys testicular enlargement would be the first sign of puberty. The testes have two major functions, to produce hormones and semen.

Other aspects of puberty include the growing of body and facial hair, pubic hair (hair in the genitalia region), changes in, voice, musculatures shape, body odor, skin changes and acne, just to name a few.