I'm sorry your baby is sick. Pnuemonia can be scary to deal with.
My ds, unvaxed, was diagnosed with atypical pnuemonia when he was 4.
I don't know if it was true pneumonia, because his chest x-rays went from showing something that looked like pnuemonia to not showing anything at all in a week and a half, which is usually considered a miracle. So the pediatrician called it extremely mild, contact derived, atypical pnuemonia, and said it was a miracle, the way he recovered.
He was off the antibiotics by day 10, and that was that. He wasn't on 'pnuemonia' antibiotics, just more of a general one, supposedly to keep him from getting worse.
I don't think my ds's vaccination status had much to do with him getting pnuemonia, other than maybe he would have gotten it worse, had he been vaxed. But that's just my opinion, formed on how well he recovered from it.
He was run down, not sleeping well, and not eating well at the time. I think that had more to do with him coming down with his sickness. He was already run down, and vulnerable to whatever was out there.
He also takes after both dh and my younger brother as far as healthwise, and dh had it when he was two and my brother had it twice before he was a year. There was no pnuemonia vax in '77, which is the year they both had it, ironically. It was just a 'bad' year. The one doctor in the office told me that she hadn't seen so many kids sick with pnuemonia (this was 2003) since the late '70's.
I think there's a genetic disposition to certain diseases, whether you're vaxed or not. All the males in my family, and dh's struggle with the lungs and the throat when they get sick. Ds is no different, other than I try to supplement him with things to help his lungs and throat stay healthy.
Again, I hope your ds gets well soon.