Have you changed or added something to his diet recently??
Is he living in a drafty cage??
Is he getting enough fibre??
Has he been on damp grass??
Has his teeth been checked by the vet?? Sticky bottom syndrome can be caused by underlying dental problems which unfortunately is common in rabbits, dwarf lops especially. It is not always just the teeth but their roots. Get his teeth checked out. I am very experienced in rabbits with teeth problems and sometimes it goes undiscovered by the vet. The only way the roots can be checked its through a xray. Donot wait for a visual sign of illness before taking him back.