I think if you can get your hands on what cafarmgirl mentioned she gave some solid advice. You might also check his crop. The rooster we have that is sick right now was sick as a younger chick with falling down and acting strangely, not able to hold head up and it ended up being an impacted...
This sounds like exactly what I logged in to find info on. We have a rooster and 2 hens who all of a sudden today started showing these signs. We've quarantined the three.