I’m guessing you mean like a big smelly shiny chunk of poops clumped together(?) which are cecals. These are fine but bunnies usually eat them. If your bun is producing excessive cecals it tends to mean they are receiving too much nutrition, but it’s common in baby buns and buns who have just been brought home, so in this case it’s probably just a reaction to the change in diet.
My buns had a few days of extra cecals when I first brought them home and changed their food. I wouldn’t worry!
It’s a pain cause they stink and are a bit of a mess to clear up, but I wouldn’t worry unless it continues to happen.
How fast did you change the food? It’s important to be careful because they have sensitive digestive systems, but it sounds like you know that!
As for his butt.. I’m surprised he’s not cleaning it himself. How much poop are we talking? It’s not really a problem unless it’s really stuck down there and causing a blockage. If it’s just a bit I’d leave him to it, he’ll probably sort it out.