J.C.
I did some research on this when I thought my 3yo had a cold sore (it turned out to be a canker sore, totally different.) My husband gets cold sores and he is very careful, but it's impossible to be 100% careful.
Anyway, what I learned in my research is that most people - like 90% - have the virus in our bodies even if it never causes any symptoms or infection. You would have to talk to your doctor about it, and I'm not a medical professional - but my guess is that your son inherited the genetic likelihood to have the outbreaks from his dad/grandma. I've been with my husband for 16 years and I don't get cold sores - but I'm positive I've been exposed, and your son would be exposed at some point in school or whatever.
I understand how you feel - I freaked out over my son even though I somewhat expected it and still expect it to happen with one or both of my kids. But it is very, very common. It's natural to blame your mother-in-law, but I wouldn't expect my husband not to kiss my kids (unless he has an active outbreak, and they understand that) and I don't think I could act like a leper around my grandkids, especially if I was on anti-viral drugs as it sounds like your mother-in-law was.