Remember use plain canned pumpkin, not pumpkin pie filling. Most dogs love it and it seems to have the incredible to soften hard stools and firm up soft stools. Good idea to keep an eye on his general appearance. A trip to the vet may be necessary should this not resolve very soon.
Agreed with Sam and Sam. Canned pumpkin has always done well. Sam T.'s question concerning rawhide is a valid one. It can cause havoc! Let us know if things "loosen up." :-)