constipation in dogs
Personally I wouldn't give him anything if you are going to the vet tomorrow.
Constipation can be caused by too little fibre in your dogs diet.
BUT it can also be due to a bowel obstruction (from eating bones which is a major no no) or a hernia.
A common reason for constipation in dogs is blocked anal sacs, which can be squeezed to relieve pressure but may be infected.
Either way a trip to the vet is the best solution.