He has a poor appetite now that I changed his food.
He doesn't seem to be interested in his new food.
この場合、次のような言い方をするのが自然で良いかと思います。
ーHe has a poor appetite now that I changed his food.
「餌を変えて食欲がない」
appetite で「食欲」
ーHe doesn't seem to be interested in his new food.
「新しい餌に興味がないようだ」
doesn't seem to be interested in ... で「…に興味がないようだ」
ご参考まで！
カナダ 
