Why is the dog constipated? Is there some kind of stool softener you can give to make it not hurt so much. Has it been to the vet?

If he is still constipated, feed him some canned unsweetened pumpkin. (NOT pie filling, just the plain stuff.) About 1 tablespoon per 20# of body weight. Works for constipation AND for diarreah. (not sure how it works for both, but it does.)

As far as getting him to go out, relax and don't yank on the leash. Try to coax him with treats and use your high pitched happy voice. Or, if he goes in the car willingly, put him in the car, drive a half a block, then get out and try getting him to go for a walk.

Please take your dog to the vet, he could have a blockage or another issue. Trouble going #2 may not be that your dog is constipated. And please stop yanking on your dog...it is unneccessary.