Does it feel like a scab? It almost looks like he may have just rubbed it a bit on his bunny dox or something..my rabbits love to rub themselves..and chew...so perhaps he was just a bit. Aggressive rubbing against his metal feeder? Just a thought..it doesn’t look major
If it's not just a scab that clears up and goes away, but persists and may even worsen or spread to around the mouth or eyes, then it could be a case of rabbit syphilis, which would need to be treated with pen g procaine/benzathine injections. http://www.medirabbit.com/EN/Skin_diseases/Bacterial/Syph_gen.html