S.H.
OMG I thought my son was the ONLY one like this.
It will go away. Their thing for armpits.
My son loved my armpits too, and it was the stubble that he liked and the smell because it was a "cozy" smell to him, and it was "Mommy."
For him it was the tactile thing about it, the stubble. He liked to rub it.
But its irritating. So I'd just tell him stop it.
He's 7 now and outgrew it.
But its a fond memory.
LOL
He also liked to rub my Husband's face when he had beard stubble.
Again, it was a tactile thing for him and he liked the stubble feeling of rubbing it etc.
We replaced that with a scrub brush, for him to carry around. It had the same "stubble" feel to it.
LOL
But he didn't get too attached to the brush.
Life goes on.
They stop sooner or later.
Sooner if you just tell them stop it.
At least my son, did NOT do that outside of the home.
He knew better.
Only at home, he did that and not around other people.
And it was only ME and my armpits, that he did that too. Because, I am "Mommy." It was just cozy to him etc.