German Shepherd Dog Forums - Ideally, how much food should Duke eat per day?

Hello all, our 3 year old male weighs about 105lbs, is very healthy and with no medical problems, and he is a very tall/large-bodied GSD. Locally we've gotten a lot of snowfall this winter but its been iced over for a while, making it no fun for poor Dukey to go romp around in, so he's been getting less exercise than usual. My concern is he might start to gain a little flab while we wait for better weather. How much should a shepherd his size/age/activity level be getting per day? He'd been on Instinct Rabbit formula for quite a while until our local feed store ran out, so we bought him a bag of Innova Large Breed to tide him over, but we're so unimpressed with how he's doing on it (he's started to itch here and there for the first time) we're putting him right back on Instinct once the bag runs out in a few more days. I've been giving him about a cup and a half for breakfast first thing in the mornings along with one scoop of Hip & Joint supplement powder and a dallop of canned pumpkin, as well as about 2 cups of his food along with same amount of supplement and pumpkin around 5 in the evening. He's not really a big treat or bone fan (I know, shocker right?) so he doesn't get any. In the warmer months he gets TONS of exercise in our large yard and on walks/hikes/hunting trips, but for right now he's mostly a homebody. Any advice on feeding?