Me and my boyfriend have a baby on the way and he's 18 and got arrested the other night for just public intoxication and trespassing 3rd degree and when he got arrested he told the cop he was his brother and went to jail and now is out on a securing bond all under his brothers name he has court coming up and called and came clean that he lied about his information how much time do you think he's facing?

That wouldn't be perjury. Perjury is making a false statement under oath. It would be lying to a peace officer.

Giving some one else's name is quite common and he may be having charges added to his already racked up criminal charges. The lying to the cops charge will just be lumped into what he's facing now. They won't make it a separate charge all by itself.