I am going to go against the grain here a little bit
Your Partner shouldn't be giving you ultimatums and on that basis, I agree with others that he is overstepping his role/authority here. He isn't helping support you, but making your life even more difficult.
However, your son is now an adult and as such, I think that however painful it may be, there may come a point, when actually, you are not helping him to sort himself out, but enabling his bad behaviour. I don't think you are there yet and I hope he does go to the therapist and continues to go and that it helps.
But I think you also need to be prepared to accept that there may be no way to actually help him and he may put you through more huge amounts of grief and angst and that in the end all your efforts are in vain. You are entitled to have a life of your own and to have some happiness for yourself too. You could end up giving everything you have to help your son - and it still won't be enough.
As I say, I don't think you are at that point yet and you may very well be able to help him. Just be aware that if it doesn't work, you need to think about at what point you will give up - otherwise you could spend the next 10, 20 years giving everything up and destroying the rest of your life - without getting the result you are hoping for.
Out of Interest, what prompted your partner to say your son has [already] destroyed your life? Is that based on knowledge that we don't have?