I'm sorry for your loss.
If you still have his body, it would be best to send him for testing/necropsy to find out what's going on
http://www.nj.gov/agriculture/index.shtml I understand you're distressed - wanting to prevent this from happening again is very good, but without more information you have no way of knowing what or how to treat.
I agree with
@KikisGirls - it may be best to "reset" or go back to the basics. Fresh food and water, limit treats to no more than 5-10% of their daily intake, make a source of poultry grit (crushed granite) available and see that oyster shell is provided free choice for any birds laying eggs. Check your birds crops at night to see that they are full and then first thing in the morning to make sure they have emptied overnight. Look them over well for any lice/mites and consider having several fresh poop samples tested for any worms, coccidiosis and bacterial infections.
If you wish, offer poultry vitamins and probiotics once a week -most feeds already contain what a chicken needs, but an occasional boost won't hurt. Olive oil
can be given in small amounts to chickens that are
sick like impacted crop or constipated, etc. it should not be given as a daily preventative - save that for when it's truly needed.
I hope all goes well for you.