This has popped up a few times here, in case you want to search for even more ideas.
One way is to give numbers of a scale that doing it in the head isn't pruductive.
Another way to come at it is from a different persective; like through programming with a setup like Scratch where he can see the value (and have to) deconstruct the steps in a process. You can also present it as a teaching tool. He can do whatever method to get the answer himself; then deconstruct the problem (after he writes his first answer) into steps as would be needed to help another person do the problem who doesn't have his skills. This is the concept that really clicked for DS8; because as much pride as he takes in his math knowledge/abilities, he has as much pride in being able to share his knowledge.
At scores in that range and the drive to work it in his head, I would hate to undermine the intrinsic motivation with extrinsic bribes.
Even errors in his head are good because then he is likely working at his level of challenge.
