Our Once AMAZING Relationship Changed Forever...

We were great for a year and half...then this month my boyfriend and I started feeling a distance.

His friend was being abusive to this girl over 7 years and they broke up a month ago, but my boyfriend is literally coming to her rescue and putting our relationship on the back burner...
After I advised him that there's no reason to be friends with his friend's ex-girlfriend, he got SUPER upset (although I have good reason - she flirted with my bf for years while she was dating his friend, so now I know she likes him deep down), he said how can I expect him to "cut friends off" - which she's not HIS friend - they were acquaintances through his friend.

Something just isn't right...Lately he says he feels "smothered" and "suffocated" and he's spending more time away from me. I have a feeling he's hanging out with his friends ex-gf because she's all he talks about and for the first time in a year, he slept at his parents house when we practically live together.
I have a feeling he's cheating on me and telling me he wants to spend "healthy time" with his friends and that that is actually code word for he wants to be with this girl and see if she's worth dumping me for.

He's so cold to me and I can't prove he's cheating. I'm also very much in love with him and don't know how to try to make things work. Everything I say he turns into an argument and gets defensive...
Should I let him go be with her to see what it's like? I've met her twice and she's a complete airhead with no prospects in life, so I'm not sure what his thrill is with her unless they're sleeping together :( It's hard to give him the benefit of the doubt when he's constantly on his phone and suddenly it's VERY important to him to spend "time with his friends." I want HIM to be the one to end it... How should I let this unravel?