Hi Janis, sorry the diverticulitis is flaring up again! Sheesh! You need a break.
We also have a rescue dog. She was very timid and even afraid of the cats (80 lb lab/german shorthair mix) when we got her. It took quite awhile for her to really acclimate and show her true personality, which is quite silly. Wouldn't have guessed that at first. It took a long time with her, partly because she was much older (7 when we got her) and we messed up on introducing the cats. They told us she got along with cats (which I think to them meant she doesn't chase them), so we didn't do the gradual intro we should have. Our cats are used to having a really friendly dog (our last dog raised one of the cats). Anyway, the cats (especially the young one) were trying to be welcoming by rubbing on the front of the dog, and that is quite rude in dog language, plus she was really scared when they came up behind her and would snap at them (very scary- thought we might have to give her back). Anyway, we ended up going to a dog behaviorist who showed us techniques for gradually desensitizing the dog to the cats. It seemed like it took forever at the time, but now they all get along, and she is quite affectionate and happy. We also had to desensitize her to being in a crate, even while we were there (part of the cat desensitizing). "High value" treats are definitely important. For ours it was string cheese, hot dogs, or freeze dried liver. Otherwise, we thought she wasn't food motivated either, but for the right food, she was! Good luck with your little guy!