Good for you for taking on a rescue dog, I have 4 and it can take ages for them to settle in so things like nail clipping usually have to wait until the dog is confident in you and his new home, could take up to 8 months if he has issues.If he is very active then give him tons of exercise,miles and miles of walking as this will ensure he is more relaxed at home., it will help you bond too to be out and about.He could be panting in the car for all sorts of reasons, some dogs just don't travel well.Try just letting him sit in the car for a few minutes a day when the car is stationary, don't make a big deal of it, then work up to maybe 20 mins then turn the engine on etc and build up his tolerance. Same with his nails,when he is properly settled in in a couple of months just gently try to clip 1 nail a day,make it a calm and nice experience with treats straight away after a nail clip before he forgets whats' happened. The most important thing is that he learns to trust you and he settles in so put non essentials off for a while, and nail clipping is non essential, as long as he can walk ok then they can wait.Good luck with him.