all you need to do is take care of yourself, your child and your unborn baby. Put your energies into them. He is old enough to look after himself. It is not your responsibility to look after him, take care of him, check if he is drinking or not, book him into rehab hope he goes etc etc etc. He is old enough to do that himself and if he wants to go he will do it.
your child and unborn child are not able to look after themselves. they need you to look after them. please focus on the people who really matter here