Looks like it's going down for Boosie's son's birthday -- as in oral sex -- but the kid's only 14.

From the "God we hope this is just a VERY bad joke" department ... Boosie wished his son, Tootie Raw, a happy birthday and promised him a bag of money and "a bad bitch to give u some head."

Here's the thing -- even if it is a joke, it's a joke about child molestation, right?

In far less troubling news ... Boosie also promised Tootie's dropping a mixtape soon.