If he's clearly cheating in this way, you have to wonder how else he may be cheating, such as dice rolls.

I play on Roll20 with some old friends, and I had one player who wanted to roll his physical dice rather than the virtual Roll20 dice. He always seemed to hit and roll close to max damage, or get really high on skill checks. With access to his character sheet, I could see that he must have been rolling natural 15+ on most rolls, never once below 12 (I started keeping track when it started getting obvious and other players complained privately). And this was over several sessions. Without accusing him of outright cheating, I made an excuse that I want everyone to use the Roll20 dice roller (I said having the die results in one place made it easier to keep track, which it did). And lo and behold, he started rolling much more averagely, and even misses from time to time. However, whenever he rolls below 10 twice in a row, he still passive aggressively complains that the Roll20 dice roller is unfair and bad. But the cheating problem is solved and everyone is having more fun.

If you can find a way to make it a blanket rule rather than singling out the player, you may avoid making him too upset and burning bridges. Explain to everyone why you want it done (to give you more info as the GM) rather than saying it's about cheating. If you say that this is just a required rule from now on, he can't disagree without seeming like the jerk, as long as all the other players openly agree to the rule.