My dog is constipated. What foods or home remedies can help him poop?

I would definitely not add salt as this can cause him to have salt poisoning and subsequent neurological problems. I would suggest adding 1-2 tsp of canned pumpkin (not the pie filling) to his food daily which will add fiber and hopefully allow him to go #2 a bit easier. If he is still having issues then I would have his prostate examined to check if it is enlarged and possible the underlying problem as to why he can't go easily. An enlarged prostate will push up on the colon, making the hole narrower so that poop can't easily pass through. The cure for this is to have him on a laxative through your vet and ultimately getting him neutered can resolve this issue for good. I hope everything works out for your pet Kitty. Take care.