Hi Jim,
Some good suggestions so far, Epsom salt can be a good treatment to use as it is for humans for constipation.
You could also use a treatment called Octozin by waterlife research here in the UK, the same company that produces the more familiar whitespot treatment Protozin and fluke treatement Sterazin.
Octozin is used for both Hole in the head and bloat which like already mentioned is generally an internal bacterial issue. I have found this treatment to be ineffective when used at the stated dose but have had a reasonable amount of success when dosed at 5 times the dose recommended.....yes you heard me right, 5 times. With scaleless catfish such your granulosus this this kind of dosage sounds crazy, generally with Catfish we would be more likely to add half the recommended dose. This treatment comes in tablet form so is easy to use, it will not upset the bio-filter either so you might want to give it a go.
Another reason could be an internal tumor in which case there is nothing can be done, its certainly worth persevering with the above first, stick him in a hospital tank with say a plastic pipe for cover and bare bottom tank, that way you can keep it clinical plus you will see if he/she has passed anything during treatment, I would have thought you could probably use both Epsom salt and Octozin in conjunction with one another.
Tweak the temp up a little also, say 28 deg C, this often helps to alleviate blockages in the digestive tract.
Good luck