I wish I had the answer for you...sometimes I am in the same boat. Michael's father and I are seperated and as far as his medical issues (which have been very few) but even just trips to pedi, staying home from work due to illness, overnight hospital stay for RSV, etc..I've done it ALONE.
Michael's father will be the first to say that drs are crooks, they just want our money, that Michael didn't need a helmet and so on. IMO it's the manly side of him just being a turd and wanting that perfect child. I am lucky, Michael has not had many illnesses and aside from his helmet for plagiocephaly there hasn't been anything major.....until now. It may be that Michael needs to wear hearing aids on both years. Testing showed that he had a mild-moderate permanent hearing loss in right ear, now (9 mos later) testing shows he has a high frequency hearing loss in both ears. He is not saying many words, but he does understand what I ask him to do etc. Dad's first reaction is he doesn't need them, he's not deaf, etc.
YOu know what I don't care what his ego says, I will do what is right/best for my son. I don't care what he has to wear, etc. I also pointed out to dad that would he rather us not do anything to help his hearing/speech and let it go and then the kid has to have speech therapy, is behind his peers, etc. He gets it but not totally.
So, I really have no advice other than make the decisions you need to. If you have to do the medical stuff alone..do it. Dad didn't handle anything well from when Michael was born...so in a sense I"m used to it. The deterioration of our marriage is not due to Michael...his father has the issues that he needs to figure out. I'm really sorry you are going through this...but, sadly you are not alone.
Jamie, you said it perfectly and that's how I handle it with a non-involved spouse.