Hello-
There are a few things you can do to help a constipated bun. My boy, Erik, has had problems with constipation in the past.
Try feeding him some greens, like romaine lettuce. If he is a young bun he shouldn't get too many, but some will be good. Also, I fed Erik clover when he had problems.
Canned pumpkin is good. Try putting it in a syringe, maybe mixing a drop or two of vanilla into it to "sweeten" it. Don't want to give him too much sugar, though!
Also if I remember correctly, alfalfa can have a mild laxative effect. But you don't want too much as that will really throw off his young little tummy.
If the condition doesn't improve, he will need to be brought into the vet to assist with his poopie problems.
Best of luck to you and your bun!
I'm also moving this to the Infirmary, where it may get some more attention.