It depends on the individuals involved…my youngest brother, 12, still kisses my dad on the lips, but my other brother, who is 15, barely even hugs my dad (I’m sure it’s just a phase.) Meanwhile, my uncle kisses his dad, my grandpa, on the lips, but my dad only shakes his dad’s hand…I’ve never seen them even hug.
I don’t think there’s anything wrong with dads and sons kissing on the lips, just the same as I don’t think there’s anything wrong with any family members kissing on the lips as long as it’s G-rated, obviously, and it doesn’t make any of the involved parties uncomfortable.