What are the exact symptoms? What do you feed? Does he free range? What makes you think the crop is involved (swollen, etc?)
Antibiotics is not a good idea if you suspected crop issues. An impacted or slow crop can become a sour crop, which is usually a YEAST infection, not bacterial. By using antibiotics it will kill beneficial bacteria and let the yeast get a much stronger hold. The ladies out there know exactly what I mean about antibiotics sometimes leading to yeast infection...
First step would be to separate out this guy into a hospital cage/dog crate, etc where you can keep tabs on him, see exactly what he is or isn't eating, etc.
If crop issues are suspected, you can try witholding feed for 24 hours and only feeding small amounts of bread SOAKED in olive oil (and water, of course). Then massage the crop to help whatever is in there to move along.
Is there a sour or nasty smell to his breath? Then it may be sour crop/thrush infection. Do a search here for 'sour and crop' and see what you can find as far as treatment options for that.
If the crop is just impacted (no sour crop) and the olive oil doesn't work he may need crop surgery to remove the blockage and get things going again.
Good luck with him...I"m just one person here so hopefully others can chime in with suggestions too.