and that's the first sumo bite to leave a mark

yikes

really might be time to see a vet about behavior issues

i kinda second that suggestion

I have an appointment

take notes on when the bites happen, what his behavior was immediately before & after the bites, and also what he was doing 10-20 minutes before the bites

like if he was resting comfortably in his tree for a while and then climbed into your lap and bit you, or if he was playing and a few minutes after you'd stopped he bit you, or so on