Well, it is hard to answer without knowing how he defines morality. Is it something religious for him, or otherwise following rules established by somebody else?
To me, morality means acting in such a way as to avoid harming others, and prevent harm that would be done to them. With that definition, staying in a monogamous relationship, if the partners are hurt by it, is what's immoral.
As for commitment, you can have commitment without exclusivity. You already commit to more than one person. You are committed to your friends, your job, your family. Commitment means that if someone needs you, you will be there. Yes, sometimes you have a big assignment on your job and a school play to attend, or a sick friend and an anniversary. You make decisions to honour your commitments based on urgency, importance, who else can replace you, who needs you the most, etc. It's the same for everyone, even with a single partner. Adding one partner doesn't make as huge a difference as many people would have you think. We already have obligations to more than one person in our lives.