I wouldn't worry much about constipation if he hasn't been eating much. (If he doesn't eat much, he won't poop much either.)
I found some differing opinions on gill flukes. But most websites agree that praziquantel is an effective medication.
API General Cure contains praziquantel. You said you didn't find it effective. How long did you use it?
It looks like there are two groups of gill flukes: Dactylogyrus
. And, according to the University of Florida website, there are FOUR different groups.....
Below is info from U. Florida's website: http://edis.ifas.ufl.edu/fa033
U. Florida recommended praziquantel:
" A common treatment method is to use praziquantel at 2–5 mg/L in a prolonged bath for 2–3 weeks..... Regardless of concentration or contact time used, small monogeneans may be protected by mucus and thus survive treatment. Infested fish should be re-examined 1–2 weeks post-treatment to determine if treatment should be repeated."
U. Florida also suggested salt baths may, or may not, be effective. Their article says:
"Use of increased salinity to control monogeneans in freshwater fishes can have variable results. Some monogeneans are more tolerant of saline conditions than others. For example, one study of two gyrodactylid species... found that 100% of G. turnbulli were eliminated when guppies were exposed to 25 g/L (ppt) salinity for 15 minutes. However, the same treatment removed only 72% of G. bullatarudis. This same study reported that increasing salinity from 0 to 3 g/L resulted in an increase in numbers of gyrodactylids. The authors reported the fish produced more mucus in response to the increased salinity; the monogeneans fed on the increased mucus and were also able to adjust to the salinity change. In contrast, some freshwater monogenean species are able to live for several days in full-strength seawater (30–35 g/L) but are not able to reproduce during that time. This wide variability in salinity tolerance of the different monogenean species found on freshwater fishes means the use of increased salinity may not be effective in controlling them.....
"Regardless of the salt concentration used, the minimum contact time is 10 minutes (15 minutes for some monogeneans—see hypersalinity in the section on treatment). However, if the fish roll over before 10 minutes has lapsed, they should be immediately removed from the dip"
So if you want to do a salt dip/bath, I can work up the numbers required to achieve a high enough dosage of salt.