Ok, if he has both types of poop then it's a cecal problem. Cecals can get out of whack from excess carbs,protein, or new veggies that they aren't used to. Sometimes it's a parasite infection, but I'd try switching his pellets first. Like I said, it's actually fairly common for adult rabbits to become sensitive to the high protein pellets they've been eating for years. Mixed pellets with the seeds and such in them are actually very bad for rabbits. Ignore what the packaging says, it's carb-laden junk food. Plus they tend to have low amounts of fiber, which can be another cause.
At the least, switch him to a high fiber alfalfa pellet.Oxbow makes a good one, or go to a feed store. Many of the alfalfa pellets in pet stores aren't very good compared to pellets made for breeders, and are more expensive too. Look for something with no treats or colored bits, 16% or less protein (less is better)and 18% or more fiber (more is better). Ideally, you should switch him to a timothy pellet such as Oxbow Bunny Basics/T (14%protein, 25-29% fiber) or Kaytee Timothy Complete (13% protein, 18-23%fiber).
Also limit his pellets to about 1/4 cup per 5 lbs of rabbit, give or take depending on whether he needs to gain or loose weight.And make sure he eats lots of grass hay. He should have grass hay such as timothy available 24/7.