What can I do?

If anyone can give me advice I would be eternally grateful. My fiancé and I have recently split up because of his constant mood swings which make us fight. After reading stories in this forum I think he may have cyclothymia. He left 3 days ago after a huge arguement over nothing as usual. The things he says to me never make any sense as to why we are arguing to begin with . They always end the same way that it’s my fault and there’s nothing wrong with him. His family tell me he has been like it for years and just put it down to plain stubbornness. I hiwever think it’s more. He’s so loving and thoughtful for about 2 months then it all goes down hill and he gets irritable, snappy with me and then will shout so much my ears want to bleed, never making any clear indications about why he’s angry. Then he locks himself away with his headphones on and watches film after film on his phone. He Rarely eats when he’s low unless it’s junk, rarely drinks and doesn’t sleep much and doesn’t speak to me, unless it’s shouting at me. After about 5 days he comes back to the man I love dearly and everything for him is normal again, but by this time I feel like I’ve been on a spin cycle in a washing machine, left wondering if I’m to blame . this continues for about another 2 months then the cycle starts again. He’s adamant there’s nothing wrong and won’t seek help. He has currently left our home and locked himself in a room at a relatives and refuses to see anyone. He won’t talk about how he feels to anyone, I’ve tried so hard to get him to talk. What can I do ?