Fur loss to the extent of seeing bare skin can actually happen naturally as a rabbit molts.  If it is a large area though, I would consider factors like mites or ringworm. How does the skin look and how big is the skin patch? Is he shedding right now heavily?

If the skin looks healthy, it’s more likely shedding. When grooming, you should be removing fur that has naturally “released” itself already, so if what’s left is skin, that’s simply the result of your rabbit’s shedding pattern this time. Grooming daily during a heavy molt is important, because if you leave it for him to eat, the consequences of a fur blockage are much more lethal than getting stressed from grooming.

Overgrooming can be done by a rabbit themselves, which is problematic and involves pulling out their own fur. This can occur when the rabbit is feeling unwell and/or stressed. Is there any changes that might signal he’s doing it himself/he is particularly stressed?

It may be just shedding then, but I’d keep an eye if the skin patch gets bigger. How big is it now would you say? While rabbits can have bare skin from molting, they are usually not big.

That sounds reasonable for a molt patch!