Not surprised he suddenly changed his mind about being groomed once you started on the (much needed) undercoat! Good for you for trying. You are right - a good bunny vet can probably get all the mats out, so you'll have a fresh start - and so will he. Having mats means he's probably extra-tender-skinned there, which isn't helping matters.

No one will think badly of YOU for getting him at a yard sale - he needed a better home, and along you came!

We feed Miss Hoppy fresh veggies all the time - she doesn't eat any pellets. She lives on produce - parsley, dandelion greens (and flowers, now that they're in the yard ...) and a carrot a day. What had he been eating? The house rabbit society pages have "what to feed your bunny" guidelines, which we have always followed, since Miss Hoppy dumped out the first dish of "rabbit food" (that pellet stuff) the day we brought her home! He needs the weight, apparently, so a carrot a day would probably do him good!

Plenty of tasty sticks, by the way, are good, too. Again, http://rabbit.org has guidelines, if your bunny doesn't have a grandpa who excels at "tasty stick finding" like my dad does for Miss Hoppy!