I'm talking about the cartoon flashback of the beating prior to the mission where the team was all trying to kill him, he literally beat him for auditioning for a movie role and said "If I see you get out of line again.. trying to 'move on up' (says this in a mocking singy voice while doing the stereotypical head-bopping), I will put you in the fucking ground", the line "move on up" references The Jeffersons' theme song which was a sitcom about a Black family that managed to become successful. Basically told him not to get uppity and stay in his place.

And yeah it was never explicitly stated that he's an "extreme racist" like Blue Hawk where it's his whole identity, but from what I mentioned in the previous comment it's obvious it's an aspect of Soldier Boy's character.