We have been lucky so far: Sophia got a cold - fever, cough & runny nose once.
I take her to our gyms daycare and she gets exposed to germs there. We also go to a library playgroup and there is always at least one kid that's coughing. I did learn from the moms there that after the group finishes, they take their kids and wash their hands. I don't know if it helps but I do that now, too.
Now that Sophia is 1 yrs old, I think it's good for her to be exposed to germs, so she can built her immunity up before she goes to kindergarten/school. But when we have plans to go somewhere I make sure we are not around sick people/kids so we can all enjoy ourselves- especially if we are planning to go on vacation.
Maybe you could just let him get better for a while so that he gets a chance to get 100% healthy and then start exposing him to the germs again.

Hope he gets healthy soon.