If this is a chronic issue then there may be an problem with his food. You should consult your vet to also make sure there isn't an internal problem that can be fixed.
If it's just an occasional thing then I suggest some apple sauce. It will help loosen his bowel movements to make going easier. Most dogs will lap it up very happily. Just make sure you don't give it to him too often as he may end up with diarrhea.
You also may want to consider cutting down on the number of treats you give him a day. If he's given them too frequently they may bind him up a bit.
If the apple sauce doesn't help within a day or so then make sure you consult your vet. You don't want to leave your dog constipated for too long as he may end up causing some internal damage if he tries to go too hard.
There are several good ways to get your dog to go. Our chow recently had constipation and we fed him a spoon of canned pumpkin in his dry dog food. We've also found high-fiber cereals such as bran flakes mixed in with their food work well, too. Fiber-type laxatives such as Metamucil will also work. Be patient because fiber sometimes takes a day or so to work.