50 Cent's Son -- I'M TEAM MAYWEATHER NOW

This one's gotta hurt ... 50 Cent's estranged son Marquise has a new father figure in his life -- and it happens to be his biological dad's worst enemy.

Marquise posted a photo to his Facebook page .... hangin' with FLOYD MAYWEATHER!!

Unless you've been living under a rock, you know 50 and Floyd hate each other's guts ... so for Marquise to post this pic -- clearly, he's trying to get under his father's skin.

50 and Marquise have a complicated relationship -- 50 hates Marquise's mother ... and hasn't really been a part of Marquise's life ... something Marquise admits he's very upset about.

Now, it seems like things have gone to "Hook" levels -- with a public war for Marquise's affection.

So far, 50 has yet to post anything about it on his Instagram ... but we're guessing it's only a matter of time.