How will you deal with a "mama's boy" type of partner?

Mamma's boys are fine but them as a partner can be quite a challenging affair because their nature affects your relationship as well. I can understand what you mean as my husband too falls in this category. I think the best one can do is not to get into his relationship with his mother at all. Keep yourself away out of what he talks to his mother or how many times he talks to his mother. But in this whole scenario, one thing that you have to keep in mind is that, you must not share everything with your partner, as chances are high that your MIL would know everything. Yes, this is a hindsight to it that you cannot be an open book in front of your partner because he would see no harm in sharing it with his mother or his mother knowing it because she is a mother.