Louhitchen, has your partner explained why he is reluctant to try antidepressants? I'm presuming from this that they have been suggested/recommended to him by his doctor?
Becoming withdrawn is very common. My husband does this. He isolates himself and seems to need his own space. I've learnt that if I push during this time, it always backfires. He is better left to his own devices at this point. I appreciate just how hard it is, but there is little that you can do - many sufferers of PTSD need their own space.
I would suggest that he follow his medication regime exactly as his doctor has prescribed it, but if there are things that you notice about his behaviour that don't seem right during this time, it is always important that you make a note of it.
I'm really glad that he is getting some help for himself - this is a crucial step, and one he has to take. There is little you can do for him, as he needs to go through this journey himself. That said, I think that having a look at what you boundaries are regarding certain behaviour/actions is always useful, as is making sure that you have the support that you need for yourself, and have interests/hobbies outside the relationship.
Good luck,
B x