This article shows you how to syringe fluids into him. https://www.backyardchickens.com/articles/safely-administering-oral-medications-to-all-poultry-and-waterfowl.73335/
Re-check his crop first thing in the morning before he's had anything to eat/drink. If it's still squishy, then I would...
What are you feeding?
I would definitely encourage him to drink the mixed corid water, but try to get the direct dose into him for a few days as well.
Do you have any antibiotics on hand? (corid is not antibiotic)
Have you looked him over really well for any injury, lice/mites and making sure...
Corid dose is 2 tsp per gallon of water given for 5-7 days as the only source of drinking water.
I would give him a direct oral dose of Corid at a rate of .10ml per pound of weight once a day for 3 days. This is IN ADDITION to you getting the mixed Corid water into him several times a day...