So I'm building up a new character, he is a Centaur Blood Hunter, and for his martial weapon, I gave him a lance. Now I know usually unmounted players using it would have to use it as a two handed weapon, but would a centaur be able to use it with the whole "can use it one handed while on a mount." seeing as I guess technically he would be on a mount all the time. Or would I have to wait for a feat like Polearm mastery to be able to wield it one handed?

It's an extremely reasonable-sounding houserule to let a centaur count as mounted for lance purposes, but by RAW, no, you'll need two hands. There is no feat that lets you get around this. Polearm Master won't help.

Ah ok, thanks for the info

So, he'd be considered his own mount, then?

And if he took the Mounted Combatant feat, he'd qualify for all benefits to both rider and mount, at all times.

This has some really weird implications I would rather avoid if it came up at my table. LOL

"...and the horse you rode in on!"

Yeah Centaurs can create some very interesting rules issues, at least they fixed it where they can't mount each other and you can create and infinite stack of centaurs. 

I read your post and remembered the existence of this video that somewhat pertains to your situation. You may or may not find it useful.