*Sigh*
It's a tough call, isn't it.
I have immune deficient children in my extended family. They are continually getting sick because of interactions with other kids.
But even so, I wouldn't isolate myself or my kids for the sake of a cough. If they have a temp or are very sad with colds, we stay home (as you've said in the OP). Otherwise, I'd say let's get out of here and endeavour to cover his mouth as much as possible when he coughs. Colds are a fact of life and I personally wouldn't (and couldn't) keep my kids home whenever they have one. (Which is all bloody winter in reality.)