Its good having your trainer ride him. I would start riding him slowly again, especially since its winter.. The changing of seasons can affect their moods.. My gelding loves the summer and can be a nut, yet in the winter I can barely get him past the jog that won him WCs hahha. This winter could be affecting him differently than the past on top of being in a new place?
I would start riding him less, like once or twice. Keep the workouts really light and noninvasive. Just basic and easy stuff, praise him a lot and keep it short.. I would say no more than a half hour.. Do things you know he likes to do, for example if he loves to jump or lope do it first.. then try getting him to relax and be supple. Spend a lot of time grooming him. Just try to make it extremely enjoyable overall. Then maybe lunge him another once or twice a week for like twenty minutes and just notice his demeanor lunging.
He may just be tense and have anxiety about one part of the workout. If you can figure out what he really doesn't like or is anxious about, isolate it and do it first. (My filly is a nutcase and fights the bit, trots to the point of forging, and is overall very irritating and unresponsive sometimes when we start a workout. Then I realized she was anxious about cantering. Because its cold, I can't really canter her right away, so I either lunge her first or I suffer through the horrible trot for a little bit, then I just canter her early and hand gallop her and let her get the ants out of her pants... I let her canter until she starts to get tired.. then I work on the trot, and she is suddenly supple and has a nice rhythm, bends around, responsive to my voice and leg etc.)
I would start by just making really easy and enjoyable workouts that seem less like work and more like bonding/fun for him. Maybe even start on the ground and let him follow you around and stuff. I'm not a believer of the horse games really, but when I ride my gelding if I need to set up ground poles or jumps or anything in the arena, I just let him follow me and figure out what I'm doing. He's practically up my butt in a cute way and I know he's enjoying it. I also give him really good grooming.. I usually don't groom him after I ride, but if we have a good ride I will give him a nice hard curry because I know he loves that.. and the positive energy from our ride comes out in the grooming.
I could be nuts though. :)
If you really think the pasture has something to do with it, I would either wait it out until he finds his place or see if you can move him. Would he be okay alone? Try him solo for a bit and see if his attitude improves. :)