I know ya'll are gay, so some of this may not apply depending on who he is due to how society and culture composes him, but applies to men in general.
Two options, you may need to alternate between them.
1: Give him space to self-isolate, which is fine for a short period. Don't go zero contact and wait for him to make contact. Touch base at least once a day and don't take it personally if he doesn't respond, sometimes even a short sterile response is too much to handle even thought he feels that he is letting you down and being a bad boyfriend in doing what he must in his state.
2: Keep spending time with him and being physically close to him if he can handle it. If you can handle him falling part in your arms, then get him to talk about what may be bothering him, but accept that depression doesn't always stem from a definable cause and not having a root cause makes depression harder to deal with. He will likely resist you being with him, just keep trying as much as he can tolerate. You should try to hold him and comfort him, bring him food he likes, and if all you are doing is holding him in a dark room, that can mean a lot.
If he can dump problems that are contributing to his state, try to see how you may be able to help him manage his problems. Sometimes being overwhelmed with problems can inspire depression due to feeling helpless and weak. Even just breaking things down into what something can be done about and what nothing can be done about, and then working together on making a plan of simple steps to manage the workable problems can help. Don't focus on the things that nothing can be done about, focus on the solvable issues.
When men are depressed, there is a feeling of weakness or being pathetic, because men are not allowed to have the weakness of depression. Struggling to "be a man" about issues feels like a personal failure and is emasculating and shameful. When you feel that way, it is normal for a guy to want to self-isolate to hide the shame, guilt, and weakness they feel. Pushing too much can cause lashing out because the pain and frustration is easier to convert to anger than to grapple with.
Make sure he knows that you are there for him if he needs you.
Therapy might be a sound suggestion because you are unequipped to help him long-term.
Sometimes it may be wise to take a break from the relationship so he can sort himself out, but that is a bit of a gamble because his feelings may get compounded by feeling abandonment and a greater loneliness, so if it gets to that, make sure he knows he can reach out at anytime and maybe reach out to him once a week to check in.