EvilCookie, I do know how you feel. My DS (4 yo) will be starting preK in the fall and I am terrified. I know he will be ill and I dread the sv invading our house.
In spite of being home with me exclusively and rarely having playdates for the last 2.5 years (since my emet went crazy in Fall 2010). and in spite of my emet/crazy sanitizing, my son has STILL had 4 cases of strep, 2 cases of influenza A, mono, croup, and too many colds to count. I just think kids attract germs, no matter where they are. Unless we put them in a bubble (and believe me, I've been tempted...) they are going to catch stuff from friends, relatives, from touching things in public, whatever. And I want you to know that of all the illness he's brought into our house in 2.5 years, we've NEVER caught anything from him except a few of the colds. I hope maybe you can find that somewhat reassuring.
Kids get sick a lot. It helps them build up their immune system, though. It sucks for them, and for us as their moms who have to worry/feel guilty about them feeling bad. As far as if you send him this spring or not, that is your call. I don't think it will hurt anything if you want to wait. I can understand being uneasy. I'm already freaking out, and my DS doesn't start until August. My main anxiety surrounds sv, but I can definitely relate.
You can PM me anytime.