Hmmm sounds like your bunny has has quite a lot of adjusting to do...
First of all, i agree, that food does not sound good. I know plain pellet may look boring and unhealthy but it's actually the way to go. The fancy mixes are unhealthy and loaded with things your bun really doesn't need.
Second of all, you haven't had your bun for very long and therefore he has gone through the stress of unfamiliar surroundings etc. The food change in itself is a bit stressful for some and might have caused a stomach upset. I'm guessing once he finished one bag you switched him over to the other kind? All that might have contributed in his upset.
Third, get rid of the chocolate. It's terribly unhealthy and even unsafe. Chocolate is a no-no that really isn't required. You should stick to much healthier choices such as fresh fruit and veggiesand only in small quantities.
I suggest going back to basics for a few weeks. Meaning, get your bun on a good healthy timothy pellet, and lots lots of good quality Timothy hay. You can pop in a bit of alfalfa in there as a treat if you want.Give him some fresh greens such as carrot tops (really inexpensive), romaine lettuce and parsley.Other than that, plenty of fresh water. Give him a few weeks to adjust to all that and to recover from the stomach upsets, etc.
Then, slowly, you can start introducing the treats. you can start with a small piece of apple. See how his system reacts to it. Some bunnies are more sensitive and react to different foods differently.
I really do think that all the stress from moving, being mistreated, and having mites contributed to it. Keep an eye on him and make sure he is drinking enough. Try to let him out to play as much as you can as that will destress him and get him to trust you.
You can also boil some rice and give him a little rice water to help firm up his stools. But not too much.
Let us know how it goes
