I couldn't believe someone would say he needed to lose weight when I had a hard time keeping weight on him during the winter! When I got him in november he was already ribby and out of shape.
Its normal for a horses weight to fluxuate during winter and summer. It is dramaticised by new life sytles to the horse too- training, moving home, new feeding regimes etc.
My horse was very skinny by the end of last winter as it was her first winter being worked and I couldn't find feed which didnt cause her ulcers to flare up. The photos attatched are jaunary ish time and now, your lee the difference!
Btw, my gut instinct is that your horse may have ulcers. It is not always the owners fault as some horses are more prone to them. I think its ulcers because the horse is sorting through his food- ie eating the alfalfa which absorbs extra stomach acid (too much acid causes the ulcers), but leaving the sweet feed which will be sugary and starchy which causes the acid to build up.
Is your horse insured? Investigations into ulcers can reach thousands of dollars. I couldn't get a gastroscope due to nasty insurance people so when straight to the management stage: No grain, only alfalfa and grass (/hay in winter), no stabling, and adding a suplement with prebiotics (promote a healthy gut environment). Direct treatment of ulcers is done using a proton pump inhibititor (stops acid secretion) called omeprozole. Very expensive, but one of the few drugs known to help.
Now for my winter/summer pics
Winter pic: you can see her ribs and there is little fat covering across the back. Very little crest too. Doesnt look as ribby as she actually was due to winter fluff and her head being down
Summer: She was slightly over weight in this pic: no ribs, slight belly, small crest and there is a good sized covering of fat over her shoulders and rump. (Shes on a weightloss scheme atm