I don’t have DID and I need advice

I’m sorry if I don’t use the correct terms I seriously don’t know what I’m doing. My boyfriend has DID and I don’t know what to do when his personality changes. Sorry I don’t know what it’s called. I especially need help knowing what to do when he’s little. I’ll take any advice one what to do, I want to support him no matter what and I just need to know how to. Thank you.