I'm presuming this is 9 months vs 12 month old, yes?
IMO, that's a hard age to start nursery and those three months won't make much difference bug wise. He'll be at the age when he'll be crawling all over, putting everything in his mouth.
I do think there is an element of truth in what your DH is saying. IME there ARE more bugs around in the winter - we're all just closer together in a warm environment. September can be lovely so the children will be outside more. Having said that though, I really wouldn't base my decision on when to return to work on that alone because you really don't know.
If you are breastfeeding that will help. If you can find any way to afford it then take your year. You won't get that time back again. The extra 3 months will make him bigger and more able to fight anything he picks up.
It all seems rather doom and gloom here regarding bugs. FWIW DS1 didn't catch anything more than chicken pox his whole time at nursery. DS2 had a couple of grim months, but touch wood has been fine for months and months now.
Lubberlich I disagree. Sure, bugs everywhere, but environments where children are in close contact are notorious for the quick spread of them (nurseries, schools).