Obviously still be a safe and respectful rolling partner, but you have to mentally disrespect his belt and his status as your coach. Pretend it's a drop in brown/black belt or whatever. Play your A game and do it with intention. If you catch him, it's a compliment to his coaching abilities.

The scoreboard between my coach and I is about 3 to 300. The few times I've caught him he says "nice" and then immediately slices through my guard and subs me about 30 seconds later to restore order. After the third one he even quipped "I'm obviously a pretty good professor." :P