When i worm my chickens my ducks and goose get wormed also since they share the same waterers, I use wazine for round worms and then I use Epernix pour-on on my ducks and goose once a year in the winter when they aren't bathing as often, that way they are getting rid of internal and external parasites at once. I don't know if worming would help but if they have never been wormed it would be worth a try, although all I have read in the Book of Geese by Dave Holderread they are pretty resistant to diseases and parasites. If i wasn't worming my chickens I doubt they would get internal worming with wazine. But just to be sure they were parasite free in the winter months I would continue to use the Epernix pour on for external parasites. Now that maybe be something you should look at, have you checked him for lice or mites on the outside? If they have acsess to bathing daily then that may not be a problem but if your like here there are sometimes a month at a time when it's too cold to give them bathing water because it freezes so fast, thats when they can get external parasites. I didn't mean to write a book just hoping to help you figure out what maybe going on. Is he off his feed at all?