How can I help my husband?

I'm to a point where I don't know what to do. My husband and I have been together for 10 years and we have 3 beautiful children, our youngest which is 15 months.

My husband has disabilities (he is high functioning though- works, takes care of himself etc)
However he does have OCD and over the past few years it has gotten bad and he refuses help. His mother and I have both tried to get him to see someone.
He will be fine for a while but then he will go through a phase of it. For example, the last 2 years in his mind he decided he was dying. He had a manic episode and he believed we all knew he was dying and that we were hiding from him. It was to a point where he started planning where he wants to be buried, etc.
Another episode that has been recurring and is recently getting bad, he thinks I cheat on him. Now on a side note, I have NEVER talked to another man nor cheated. I have never given my husband a real reason to question my loyalty. He just makes reasons in his head.
For example, he’s on a kick about how he thinks I have a secret phone to talk to men.
It’s to a point where every night I go to bed, he searches and goes through my phone and tries to find nonexistent evidence.
And I have to wake up to him questioning me at 3 in the morning.
It’s also so bad, I really have no friends in my life except my mom. I’ve had friends in the past but he runs them all off (thinks they cheat on their husbands and a bad influence on me)
I had a best friend at a job I worked at and he flipped out at me and it got so bad I quit the job because he decided and told his whole family I was a lesbian with my coworker.
I am not a lesbian nor do I cheat.
There’s been multiple instances of him accusing me of having thinking I’m secretly in love with his friends.
I’ve been putting up with this crap for years. Now here’s the last straw-
He is now accusing me of our 15 month old not being his. He’s accused me twice now and blames his OCD. I told him we can get paternity testing or whatever. But this child as well as our other 2 look identical to him nor have I ever had intercourse with anyone else since we have been together!
But he thinks I’m crazy for being offended and to be honest I don’t even want our baby around him now because he thinks she isn’t his anyways.
I will add to this post, he had a traumatic childhood with his dad cheating on his mom, divorce, and all the drama.
But it’s no excuse, I am a good woman, wife and mom and this man finds excuses not to trust me. And I can’t take it anymore. I’m to a point where I almost hate him after 10 years into this marriage.
I know everyone’s going to say to make him get help or leave. But it’s not that easy with him.

I would tell him he needs to get help or it’s over. There’s no reason for him to literally be trying to find non existent evidence of you cheating, & now bringing your children into it. He’s looked through your phone, you told him you agree if he wants to do the paternity test so what else does he want??? He needs help & I would tell him he needs to start asap or you will be leaving.