on the outer limits of advice, I offer this:
there is a self-energy around the head, that people getting into it is really an invasion ... more for some than for others
I was instinctually defensive of it before my last ... but with my last I had a situation with a child we see regularly who would NOT stop messing in his headspace, and I really had to sit back and analyse because snapping at someone else's child for something that parent thinks is reasonable ... well, I needed to decide if I had a hangup, or if it was really as NOT OK as I felt it was.
It was really as NOT OK.
And it was especially interesting that as soon as this little girl had been doing it for a bit, my daughters started doing it too ... where before playing with their brother had been OK, they started being the 'wrong' way about it, in imitation. So I had to stop them all.
There is something, particularly, about boys, and women(/girls) trying to define them, and working in the headspace of the baby is totally an instinctual way to do that--to gain control over something that they are instinctually threatened by, or that they are used to having control over but sense that they don't ...
it is also love and snuggles, don't get me wrong ... but how many adult women try to control the men around them with love and snuggles (and control?) ...
anyhow, again, the outer reaches of advice here, weird, whatever, but very serious ... your son's headspace _is_ your territory, inasmuch as any woman has a right to it, and if you sense other women are impinging, you are sensing something real. Some women will be able to kiss him without imposing ... but if you feel the imposition, it is your job as a mother to protect your son from whatever is being imposed.
Good luck figuring out less awkward ways to deal with that ... but if it has to be direct, be direct.
(I was lucky, because I've always for hygenic reasons told my kids not to touch baby-faces, so I could just extend the rule to 'heads'--and let the other kid know that it was a 'family rule'.)
Related thought--if you 'live into' your mother-right, other women cannot mess with your children as easily ... it's a faith and confidence thing. For me, I'm Catholic, and really examining, "God must have given me the authority to protect him, because I am his mother," and _owning_ that, was what I needed to do.
If ANY of that makes sense ;).
God bless.
--K.