I've read many books that posit it is very natural for men not to want to have long, difficult discussions. It is challenging for them. They are of a "fix it and solve it" mentality and a long discussion does not do that. Whether you believe that to be a general truism or not, it sounds like it might be the case here.
Definitely even if we take that as a given, he should not be talking over you. His being uncomfortable does not mean he should "stamp on you" in essence.
So try starting small. I know it sounds a little contrived but ask him for just 15 minutes to talk, and have an actual timer. And ask him to please let you finish before he takes his turn. Hopefully he should be able to agree to do this for just 15 minutes. If you give it a finite length, and clear requests, that can help a lot. That way he doesn't feel like his only way to escape is to drown you out and force it to end.
