If it were me, I'd try mixing seachem metroplex in with his regular pellets. You might be able to get more metronidazole in him feeding the pellets he's used to eating. Of course, he might not like them any better than hexshield but it might be worth a try considering a tube of metroplex doesn't cost much.
There are instructions on the label for dosing food. I believe seachem suggests one measuring spoon of metroplex (measuring spoon included) per one tablespoon of pellets. It depends on the brand of pellets used, but this yields roughly
10 mg/g food assuming all medication is soaked up by the pellets.
Dr. Noga's treatment for food was for
25 mg/Kg of body weight/day for 5 to 10 days or
100 mg/kg of body weight for 3 days, and the paper
RD.
linked to says:
"Metronidazole can be administered orally in feed at a dosage of
50 mg/kg body weight (or
10 mg/g feed) for 5 consecutive days."
According to seachem one measuring spoon of metroplex is 125 mg, so mixing that into 12.5 grams of food should give you ~
10 mg/g.