The only other option that I know of involves using EPSOM SALTS to soak him in for 1 hour (1 teaspoon in 5 gallon of water). It has a laxative effect on fish, I am told. I know it means moving him but if he has the problem it will be worth it.
You said he isn't afraid of the net so maybe he won't be too hard to move for his treatment. I would see if he could get interested in eating a veggie flake or something veggie about once a week. Since you have other fish you could try one of their flakes if you use veggie type on them. Mine are not receptive but I know they love the peas.
I am sorry that I don't have any more ideas for you. If you could get him to eat any fruit or veggie (carrot, apple, green bean, zucchini, etc.) it would be as good as peas. Good luck and let me know how it goes.
Thank you for asking 0morrokh. I think I was being overworried... I don't think he's constipated anymore (or possibly ever.) And I believe his fins are showing regrowth since I started adding the melafix. Today was day four so three more days of treatment and then a 25% water change and if his fins aren't back to normal I think I'll keep going and add some pimafix for good measure. ;D