My horse is getting skinny is it because he's an older horse?

I have a 19 yr. 16h Thoroughbred / Quarter Horse and I event him but lately we've only done stadium and dressage and we're only going Elementary and Beginner Novice....so far we've only been to 2 shows this summer and he is usually riddin 3-4 times a week.....He is an older horse and he has lost a lot of weight.....when i 1 st got him at the end of last summer he was so fat he waddled and now i got him slimmed down but u can see his rib cage and his hips are sticking out a little and above his eyes those little slopes have sunkin in....He gets feed a full scoop of Sweet Feed morning and night and he is in a big pasture with a life time supple of green gr and he has no health problems my instructor says its just old age but im worried any advice? (i have upped his grain many times and tried changing it )