A couple of thoughts. If he’s not eating his grain and is just picking at his hay, it could be dental problem or ulcer issue. If you haven’t had him checked for either recently, something to consider. You don’t say how old the gelding is, if he’s an older citizen, I’d lean more towards a dental problem.
Have either of the food items been changed recently? Even if it’s the same supplier for both, if its a new batch it could be a different grain formulation or hay from a different field. Put some molasses in his grain, maybe chop up some apples and shred some carrots to try and encourage to eat it. When my horse periodically goes off his feed, I mix all those things in with his grain and coax him to eat it, literally hand feed him. Try some hay cubes.
Something weird happened with my horse earlier this winter, he literally could not pick up anything with his “lips”, he couldn’t grab treats, it was like they are paralyzed. My sister said to check for any kind of bites or injury. He didn’t have any and the situation resolved in about a week. Could be the gelding got into some kind of plant material causing an allergic reaction of some sort which also might help to explain his being mopey.
Has he had a recent vet visit for vaccines? After my gelding is vaccinated,he goes “off” his grain for a week or so, and doesn’t eat as much hay. That’s normal for him and I don’t worry about it.
Last year he got a foot abscess and didn’t eat his grain while that was in progress, but, afterwards he was back to eating normally.
And, it could be as you say, the winter blues. Maybe do some ground work with him, introduce clicker training or other things to get his mind going. Play “games” with him. If the situation doesn’t resolve itself in a week or two or gets noticeably worse, call the vet.
Keep us posted on how it resolves.