If he lost a certain % of body weight then he should be fed however possible, otherwise I would wait him out. If he has eaten F/T before then he knows it's food. They're very smart when it comes to basic functions such as eating. I think 10% of his body weight is what people usually recommend? You'd be surprised how good they are at not losing weight when they stop eating. This is otherwise normal behavior for the season and I wouldn't get too worried UNLESS he lost a certain percent of body weight or has other symptoms.