If there's nothing seen from x ray, may be there's sprain on his muscle or the worse possibility is there's cruciate ligament. May be u should bring ur dog to vet surgeon especially Orthopaedics surgeon to get examined again. Some vet often suggest for surgery because that condition is so painful and getting worse if u ignored it day by day. As i knew before pain killer and anti inflamatory drug such cartrophen injection can help to relieve the pain for a while but can't be given for long term.