dont up the concentrates and reduce the forage, it should be the other way around.He sounds to me as it he was stressed being left behind and if he is happier in the field when the others go out, then just chuck him out. You mentioned earlier you gave him 2 slices of hay in the evening previously? that is also not a lot. I would speak to one of the nutritionist lines and would also start to weigh your hay, which takes about 2 secs to do. check under his rugs and if he feels cold then rug accordingly, don't focus on the logic "he has a thick coat, its only x", see how he is, does he move around his field a lot? there is sometimes no rhyme or reason about ruging. I have had 2 fully clipped and similar horses, in boxes next to each other. 1 stayed in a middle weight till nearly December. The other was in a heavyweight with another liner.
I have previously owned a massively stressy TB who was a very poor doer and the only way to keep weight on him, regardless of the supplements and mega bucks feed poured down his throat.. was to keep up his forage intake.