Hi, L. --
I agree that kisses on the lips should be reserved for a very limited number of people in your child's life -- grandparents, parents, siblings... that's about it, I'd say. In general, I haven't had to deal with that too much, so I don't have great advice, other than to perhaps "head things off at the pass" and say, as someone moves in to kiss him "OK, baby, ready for a kiss on the cheek?" or "Yes, he has wonderfully kissable cheeks" or "He likes cheek kisses" or something to that effect.
Since little kids are also subject to every cold, flu, and virus known to man, you could always fudge and say "He's coming down with something. How about kissing on the cheek?"
I think children should know that it is fine for them to have their own boundaries of space. My daughter likes to be a space invader with her smaller friends, and I am always reminding her that she needs to give space. The same is true with adults and children, so I think it's very conscientious of you and very wise to be thinking along the lines you are thinking. These days of sexual predation are nothing to ignore, and so, in as gentle a way as we are able, I DO think it's important to start educating our children about who can be in what kind of proximity to them and that it's ALWAYS OK for them to have their own personal space and boundaries, especially from people who are not in their direct, immediate circle -- even extended family.
Also, sorry to say it, but cold sores are communicable and people can also transmit their own cold or other crud to your child, so they really should not kiss him on the lips anyway. I was always concerned when waiters or waitresses would fawn over our daughter because I knew they'd been carrying people's dirty plates, etc., and I didn't want any unnecessary exposure to people's funk to be foisted on my daughter.
Caution is a good thing, and respect for the little people is a good thing, too!
Good luck with little Mr. Kissable! :)