Before we can determine why your horse is losing weight we need to know what he's eating and how much (hay, grain, supplements), has any of that feed changed in the last 3 months, what his living conditions are (stall, pasture, dry lot), if his living arrangement has changed in the last 3 months (including pasture mate changes), how much exercise he gets, when the last time the vet floated his teeth, when was the last two times you dewormed and with what product, and if he's shown any behavioral changes.

After all of those questions are answered thoroughly, we can get a better idea of why he's losing weight.

Also, have you consulted your vet on this problem? If so, what did he/she say?