I don't think the point here is that the affair is with his mother-- or whether that is inherently right or wrong. The point here is that your boyfriend has been cheating on you for months, exposing you to possible STDs, lying to you, and disrespecting you. He only told you because he wanted to get you involved in a threesome. I think the biggest issue here is why that was something you were willing to accept? Why was that not an immediate deal-breaker? I think that means you are also may benefit from some self-reflection and counseling. Anyone deserves to be treated better than that.

As for GSA, I am very familiar with the concept (from an academic perspective; not a personal one). I think the problem in your boyfriend's case is not that he has these feelings-- but that he chose to act on them in the way that he did. Rather than seeking counseling to work through his feelings-- or even entering into a loving and truthful relationship with his mother-- he chose to act out in a destructive way, lie to his sexual partners, etc.

Clearly, there are some self-esteem issues here on both ends. I think each of you probably need individual therapy to figure out what is best for each of you, separately.