It is so difficult to say with an unknown ailment. Treating for parasites and constipation at the same time would effectively kill two birds with one stone, if it is parasites the medication will work and the peas will clear out dead parasites. If it is parasites and you treat for constipation any bacteria in the gut caused by the parasites will be flushed out and the fish could appear to get better for a while. Personally in this case I would treat for parasites as a precaution and if that works and he improves then we are good to go at finding them a bigger home. If that doesnt work and constant water changes show no improvement we will have to think again.