I completely understand where you're coming from on this, as I am a person who has mentally ill loved ones and who is mentally ill myself. I think it's possible you could do this, though as others have said, it may interfere with his free will. If you think it's for the better, then it's up to you to decide whether this is in align with your own moral compass. You'd also need to consider what would happen if he ever found out, and if you're willing to face the possibility that he will be angry and leave you, even though you had good intentions.
I guess it all depends on what the PTSD is caused by, but as a person who also suffers from that, I can say that as much as my memories pain me, I'd never want them taken away. That suffering taught me so much about life and how to love others all over again. I would not have become the person I am without that trauma. Instead of deleting those memories from my mind, I've chosen to face them through therapy - and I gurantee your husband will get better results that way. When I tried to forget my trauma, I only buried it deeper and repressed it, which in the end made everything worse. When I faced it and dealt with it, I actually got better. I'm not one of a kind in that regard. PTSD requires medical treatment.
If you decide to go through with this, you should confront him about it. It will alleviate stress on both sides. If he's not open to magic, and you're positive there's no way you can convince him, as I said before - does that align with your moral compass? Are you willing to bypass his wishes with your good intentions, or will you help him get the help he needs, realize he knows his suffering better than you do?
I apologize if this in any way came across as offensive. This is my opinion based off of my own experience. I hope it helps, and you can message me whenever you'd like about this issue.