My (25f) brother (21m) is getting overpossesive with his (21f) gf and idk how to prevent this

TL; DR: I need advice so my brother stops getting abusive (controlling jealous and possesive) with his gf

My brother and is girlfriend have been together for 5 years. She’s still a student, and he didn’t get into college and is supposed to work.

He had kinda important health issues the past year, so he had to stop working, his work being physical and the health issue being troubles with his hand strength.
He doesn’t want to get back to school and find another way to work this out, and doesn’t really look for another job that can please him and being compatible with his hearth issue for now (it’s been over a year).

His gf is always supportive of him and trying to help, but the past month have been rough for them. He is getting more controlling, possesive, and jealous, accusing her of cheating when they’re not together (she’s studying in another city). She told me about it, and about the fact that he gets mad at her for not spending every second she as with him.
She’s working on weekends, have to study at home, a horse to care about, and does running.
She doesn’t put him aside of all of that, and offers him to join her as much as he would like to.

My brother isn’t working at this time, so he doesn’t have any occupation during the days.

He told her that’s not enough, and don’t believe her when she says she want to spend time together, but that she already does as much as possible.
She offerd that they go to therapy together to work this out, but he didn’t want to, accusing her to lie about wanting to.

She is the one worrying for him and their relationship, and told me about it. The past week, he’s been really demanding, asking for proof of what she does and with whom at any time of the day, and she can’t deal with anymore.

I adviced her to put some distance until the weekend comes, since I’ll come back in the city, hoping I can see both of them separately and have a serious talk with my brother being abusive.

They talked about it last night, and he agreed that he should go to therapy.

Since he never admitted being in difficulty with mental health and needing help, I just wanted to know if some of you would have some advice and resources to handle the situation?

I really want to be here for his gf (already told her to breakup with him sometimes when he was being a dick), but I know breaking up would not possibly change the way he feels and started to think.

Besides getting a job/a hobby/ something else to think of other than her and the fact that they’re appart all day, do you have any advice?
He hasn’t always been like that, and I’m hoping that I can help him getting better.