I would stop feeding it scratch, there's little of nutritional value in it. Flock raiser is fine, you might want to add some additional niacin in the form of Brewer's Yeast but if you're not seeing any leg problems that's not 100% necessary. I would also let him out on some grass to let him graze - grass is very beneficial to them, it's what he would be eating all the time if he were wild.
Yep, if it's getting more feathers in it's likely to be getting hot. I'm not sure where you are but even here in New England the weather has been scorching (for us at least) and my geese are practically living in their pool right now to keep cool. They get pretty sluggish and don't want to move much when they're hot (just like us). Try adding some electrolytes to his water to keep him hydrated (Sav A Chick is sold at most feed stores) and you could also try a vitamin supplement like Nutri-Drench, which as a bonus also has niacin in it.
Glad to hear you can keep him! I was thinking you took him out of the wild but it sounds like you bought him which is perfectly legal and fine.