Best Food For an Arthritic Dog?

I have a 16 year old red Collie. He is in very good health apart from some arthritis in his front legs, and rather odd looking bowel movements. Quite often he produces faeces which is bright yellow and runny. I changed his diet some months ago to Senior tinned dog food as I thought his food might be too rich for him. The problem still occurs. He will not eat dry food. Please can you ...