What you're seeing is excess caecotrophs. A bunny will produce these daily but should re-ingest them immediately and seeing them lying around is indicative of a problem.
The first thing I would do is get your bun to the vets just to rule out any joint problems, arthritis or pain can cause them to stop reaching down and ingesting their cecotrophs.
After that's ruled out, I would look very carefully at his diet. Too many rich foods can cause excess cecotrophs, such as pellets and vegetables or too many 'treats'.
I have one bunny that has a real problem with this, over the years we've tried all sorts including pain relief, a hay only diet, experimenting with foods and now it's just a case of managing the symptoms with careful diet and daily pro-biotic pellets (Protexin pro-fibre). He still has flare ups, but we know how to manage these now with the pro-biotic pellets and, if need be, the powder (Bio-Lapis).
Good luck!