Rabbits usually produce cecotropes at the same time each day, often at night. They are sometimes called "night droppings", but in pet rabbits, it is dependent on their daily routine and feeding times. The rabbit eats them straight from the anus so in a normal situation you will not see them. The only clue that your rabbit is producing them is seeing your rabbit duck its head down under its body and then sit up chewing.
A diet too high in carbohydrates, protein, or sugar can upset the balance of bacteria in the caecum causing the production of too many cecotropes. The rabbit ignores these extra dropping as they contain unneeded nutrients and they become stuck in the fur or squished on the floor instead.
This is most commonly a result of too much dry food or too many processed treats. In which case, you can resolve it by reducing the amount of pellets and increasing the amount of hay or feeding higher fibre/lower protein pellets. Here is a food comparison chart for rabbit pellets:
http://www.therabbithouse.com/diet/rabbit-food-comparison.asp
Healthy diet:
http://www.therabbithouse.com/diet/components-rabbit-diet.asp
Several conditions can cause a physical impediment to eating cecotropes, including arthritis, an injury, or obesity. These all make it difficult for your rabbit to bend correctly to collect them and they will generally ignore them if they fall to the floor. Dental problems and illness can also affect rabbit's desire or ability to eat the cecotropes. I would first suggest lowering the amount of pellets and treats he gets and offering more hay, hopefully that helps
He is getting older, and since he hasn't had a change in diet it could be something else, not related to his diet like stated above.
