I've been through exactly this with my ex, and a work college is put his DW through it as well, my work colleague realised he had to get help and stick to it. My ex didn't take that path.
You are in a difficult place OP, I would suggest you talk to your GP and find support groups to help you, I found a support group and it has been if enormous help.
Issues like this, that go untreated, only get worse over time. It's not like a cold, where some bed rest and fluudshrlp you to get over it.
Put the fact that he kept it from you behind yourselves and look at what can be done now to ease things for you. Talking to professionals that can give you an insight into his illness is helpful. I feel compassion for my ex and her bipolar disorder, but because it goes untreated and causes difficulties it is still difficult. And that my ex won't evidence any form of treatment (even to the court), just make it harder.
I will say, don't let yourself get overly angry about it, you are allowed to feel angry and frustrated, but you have to take care of yourself to so that your not pushed over the edge, as happened to me and nearly to my colleague's DW.
You cannot force him to get treatment unless he becomes a danger and then the authorities take over, police and adult mental health. Your DP needs to realise that that could very well happen if things get really bad.
I feel for you OP your are in a really difficult place, and I'd really recommend reaching out for help.