Is it permissible to assist in the marriage of a man who is not religiously committed and has bad morals, especially with the fear that he might wrong his future wife?

You should advise your brother and supplicate for his guidance. If Allah guides him through you, you will receive a great reward.

If the advice is not effective: If his behavior leads him out of the fold of Islam, then do not help him marry Muslim women. If he is not engaging in acts that take him out of the fold of Islam, then there is no objection to helping him marry; perhaps the stability and circumstances of family life will make him abandon his misguidance.