Not even close to an expert lol but I am very passionate about them and keeping them, so I have been learning from the experts for years and the guys here get the credit for my knowledge, they set me on the right path to begin with. gar just are different then your "typical" fish and once you accept it and learn how to "meet their needs" they are some of the heartiest fish I've honestly kept. They aren't cichlids... and most of what we learn in keeping fish from "the beginning" centers around housing tropical community fish and/or cichlids... and gar are very different animals altogether.
let him finish off his feeders.. then offer pellets the next day at his size he'll eat every day to every other day. when the feeders are gone for a day or 2.. offer pellets... the next am give him a few more feeders... wait a day or 2... keep this as your reg feeding routine and in as little as a week he'll be eating both w/out issue ( sounds like he may take a few though lol). I would also try softer pellets at first like hikkari, or the freeze dried krill. also ere on the side of smaller pellets. I also will toss the pellets in and go do something else and come back an hour later and remove anything uneaten... I had one for the longest time would eat pellets... but not infront of me, I also found as I increased my gar troupe that the older gar already eating pellets taught the new ones sometimes in a matter of a day or 2 to eat pellets, some would be finicky and only eat the live for weeks.. so just be persistent and keep trying different things.