Get him out. From what you said i think this is the best option, people like this rarely change and I doubt this is a one time thing. You need to convince him to leave while he still can, he is still young and probably doesn't have too much she could take in a divorce. If he stays she can not only hurt him more physically but will probably try and make sure he can't leave, which means stuff like threatening to report him for abuse and baby trapping him. Priority 1 is to get him out of there ASAP but if you're able to get evidence or have him get evidence of abuse it could lessen what she takes in the divorce, remember this is secondary, If he is willing to leave but not get evidence on her and report her you need to drop the monetary problems.