Mega colon is a completely different problem than excess cecals. With mega colon you will see abnormally shaped fecal poop(oblong, log shaped, small/large) and sometimes odd looking cecotropes. With excess cecals they are producing normally shaped cecotropes but just not consuming them. And this can be because of things like excess protein in the diet if they are getting too many pellets or their hay is too rich(eg. alfalfa hay), from being overly distracted(young bunny, new to the home, predator or something in the environment distracting them), from a health issue like obesity or arthritis making it difficult to reach down and consume them.
If the uneaten cecals are not normally shaped buy are coming out semiformed and/or pasty or watery, then it could be a dietary issue. Since your rabbit is already on a low amount of pellets and greens, it could be he is just extra sensitive and doesn't need them in his diet.
https://rabbit.org/intermittent-soft-cecotropes-in-rabbits/
If he is actually having odd shaped poop and does have megacolon, that can also make him very sensitive to his diet. I had a megacolon bun that couldn't tolerate any pellets in his diet and only a limited amount and type of greens. Plus his hay quality had to be a very particular consistency or it would cause increased poop irregularities. He had to have a medium coarse grass hay. Any softer and he would end up with log shaped poop. He was on a no pellet diet with a medium coarse grass hay(timothy), and limited greens. This is the diet I found worked best for him and managing his condition. He lived to be 5 yrs old.
https://www.vgr1.com/megacolon/