Often it is attributed to boredom, but i think it can be addictive for some rabbits also.
Barbering is tricky to try and curb. If it gets really bad, some owners have had to temporarily separate rabbits so the other can recover.
Some ideas Ive heard of to deter the one grooming is to put a little bitter apple spray on the fur (not sure how it spreads but I’d be afraid of it stinging the raw area). Another was to put an appealing essential oil on the hind end on the rabbit. Apparently aniseed is attractive to a lot of rabbits.
I wonder if securing a salt lick (not mineral) on a cage or pen wall would appeal to him. Rabbits don’t need mineral or salt licks for nutrition but a salt lick may be appealing for him so he can continue the licking action when he needs to. They are supposed to self moderate salt intake but make sure there is plenty of water available. Water will take excess salt out of the body.
Do you have some levels in their main habitat area? Some levels they can go up and down can provide some extra interest but also allow one rabbit to get away from another if they need to.
I think the overgrooming is something you will probably (hopefully) see abate by itself as he settles post-neuter and matures. Fingers crossed!