Has he ever given you a reason to distrust him previously? If not, accept what he is saying as true. He came to you instead of cheating, he is being patient while you process, and he has stuck with you for this long that it's clear he loves you and wants to be there with and for you.
Some people aren't poly. If you can't share him that's fine. Don't blame your trust though. Find the real reason. You're either jealous or selfish or just don't believe that's how things work. All of those reasons are acceptable. Not believing the man you love when he tells you his feelings isn't and it will hurt both of you.
"Regret for the things we did can be tempered by time; it is the regret for the things we did not do that is inconsolable." - Sydney Smith
Kyle: 27 year old male
Katie (rymmare): 25 year old female
Kids: girl: 5 years old, boy: 3 years old