Yes Islam is all for marriage early.
HOWEVER, he needs to be in a position to support his wife and family…Islam also talks about a wife’s expectations from her husband…her rights. If he prays five times a day, has a beard down to the floor but cannot fulfill his basic Islamic responsibilities towards his wife and family…what kind of a Muslim is he? What he is asking for is also Islamically wrong because he is in no position to take on the responsibility of another human being. Use Islam because that is important to him.
Furthermore, what would your mother have done? Think from that perspective because you are now his guardian/parent…you have to guide him and stop him from making bad choices now.
Good luck! You’re a great older sister!