This can be a sign of a lower urinary tract problem - either an infection (in which case you may notice some blood in some of the pee when you scoop) or crystals - which you can only tell by getting a urine sample to the vet, or taking him to the vet for the vet to get a urine sample from him. When Tuxie had crystals, he was constantly cleaning down there.
I really hate to be alarmist - but do try to keep an eye on him. The problem with crystals in male cats (they have very thin urethras) is they can become blocked very easily - and if they're completely blocked, they've got 24 hours before the urine backs up into their system and kills them from the toxicity. However, that said, if he were completely blocked, he would be in so much discomfort that he would very likely be in the box CONSTANTLY - or even at that point trying to go outside the box. You would most likely see him straining to go. Ever had to go to the bathroom really badly? It's not something you'd deal with by trying to take a shower or bath, you know?
So keep an eye for "little drops" or very small sized pees in the litter box. This can be a signal there's a problem.
If he keeps up the constant cleaning, he may need a trip to the vet. If he has struvite crystals, they can be dissolved with a pill called Albeta (which changes the pH of his urine). If he has calcium oxalate crystals, they can only be dealt with by changing his diet - and no more cat grass or greens as treats.
We have six cats - three are male. One had a problem with struvite crystals, two had problems with calcium oxalate crystals. We put the entire household on the diet that the two males with calcium oxalate crystals needed. It's done the trick, and we haven't had any problems for over a year now (thank goodness!)
Here's hoping it's just a temporary cleanliness urge!