I'm having white stringy poop with my FH and just read up on Epsom salt treatment, it seems pretty harmless and it's cheap and readily available at any pharmacy.
If he eats pellets it may be an option but maybe you could soak some frozen foods in it over night for a similar effect.
Basically it's a solution of water with 1 level tablespoon of Epsom in 500ml of water, you soak the food in it and feed for 5 days.
"Rd" the member done a sticky on it, when in the gut it's a very effective wormer.
Make sure it's pure Epsom salt (magnesium sulfate).
Worth doing with any new fish really. Esp wild caught.