Hopefully the tylan made him lose his appetite. I wish I could help you on this one, I've only had chickens a few years and have not encountered anything like that, altho I have noticed empty crops when one would not feel well but no fatalities from not eating. I notice my main roo barely eats some days because he is too busy watching out for the others, and when I give him snacks he gives it away, he has got pretty thin at times. If you can't get him to eat you can force some food, just not too big of bites so he can get it down properly, if you use chicken feed let it soak in water a few minutes first, sometimes that will be enough to get them to start eating again, and I would def go with a little sugar water about half teaspoon added to half cup of water. If you have not wormed them within the last 6 months I would definitely do it because they could have weakened immune systems from parasites. There little systems are so delicate, and yet they can be hardy little buggers. Please let me know how he progresses.