Babies (up to 7 months) diet should consist of unlimited pellets (alfalfa based), unlimited alfalfa hay, and water…. thats it! Their tummies are very sensitive so no treats, no veggies, no extras. When he gets to be about 5-7 months you can start the transition to timothy based pellets and hay, and then begin to introduce veggies (do it slowly just one new veggie per week- to see how he reacts).
What is in the “mix” that you are feeding him? My guess is that there is something too rich in it that is causing his tummy to be upset. A agree with jerseygirl also to get him to the vet and have his poo tested. Parasites are hard to get rid of if you let them go too long (and they can in extreme circumstances be deadly).
I have seen a stray cecal or two from my bunnies every once in a while (maybe once every other week). But the stray ones might be because they got startled or interupted when they were eating them. Because you are seeing them so often I would think that there is a diet or medical issue.
I hope donut feels better soon!!! Welcome to the forum! 