You might need to change his food. I would definately talk to your vet first, but sometimes some foods just don't agree with some dogs. Amos could not eat Eukanuba when he was a puppy. It made him so constipated he started throwing up!!
You can also ask your vet about giving him a little bit of milk or yogurt. Lactose really helps to break up a dog's stool.
But please, ask your vet what he/she thinks before you try either of these things, as I am not a vet and they will know what is best for your dog. These are just things that worked for me.
Good luck and I hope your baby feels better!