It is very normal for a child that has been home to start catching lot's of bugs once put into a preschool environment for the first time, IT IS A POSITIVE! May not seem so but he is building up his immune system.
My son did the same thing last year, went to preschool and had like 10 colds, stomach bug, you name it, however this year he has not missed ONE DAY of Pre K!
To homeschool out of fear of germs I think is extreme.
He will learn so much being in a class setting, around his peers and to be honest that is what preschool is mainly about just getting him used to listening and being in a class type setting. I wouldn't worry about what he is learning if he goes only ONE day!
You cannot keep him in a bubble and kids not exposed to this stuff, if they get sick later it could be worse. Even a trip to the grocery store could expose him, play area, parks. So you would have to keep him inside all the time to prevent it, that isn't good. It is a pain for us parents and nobody likes their baby sick, but they need germs to get strong immune systems. I wouldn't home school, kids need social interaction, to break away a little from their parents and have something of his own!
Do not rule out public school because of germs it is part of the process. We all went through it, he will be fine. Also just ask the policy on bringing kids to school sick, what is allowed (like our schools have very specific guidelines), do they wash hands frequently, do not let him drink from a water fountain but send a water bottle. Just simple things like this will help him.
I do know my daughter had a tough time though when she was younger getting every single bug out there when around kids, ended up needing her tonsils and adnoids removed at 3 and after that never gets sick. Kindergarten she missed six days total, first grade three days and this year in second ONE DAY missed, that is it! So they do build up their immune systems.
So if it seems something unusual to you, more then just the sniffles or whatever from being around other kids, then take him to an ENT and see if something else is going on.