Teddy was horribly constipated when I first brought him home and I asked at the vet, but since he didn't have a blockage or anything they told me to just keep his diet as regulated as possible, only feed him once in the morning and once in the evening and only leave the food down for 20 minutes at a time. and don't make any food changes. IN addition to that, I was told to use ONE TEASPOON of Canned Pumpkin (unsweetened all natural like you would use for a pie) once a day. (he was about 2lbs at the time) He absolutely loved the stuff, ate it right off the spoon.
It worked great, but it took about a week.
I know its heart breaking, he used to cry everytime he pooped, sometimes it sounded like a cat's meow,
but it got better and he's back to normal now.
I also had someone tell me to try baby suppositories, although I didn't need to get that drastic so I can't say if it works.