A fecal consists of checking the feces for parasites. You can either do a smear or a float. A smear simply consists of smearing some fecal material on a slide and examining it under the microscope. A float (more likely to detect all the parasites) consists of putting a pea sized piece of feces in a tube of a solution with a certain specific gravity (I use fecasol, you can get a gallon from the vet's for ~$16). You mash up the feces in the fecasol, fill the tube to the brim and place a cover slip over it. The parasite eggs and cysts will float to the surface and after 10 min you pull the cover slip making sure not to dislodge the drop of hanging fluid, and examine it under a microscope. If planning to have a fecal done, it is important that the feces be as fresh as possible (they need to be less than 24 hours old for the best results). It helps to put the fresh feces in the fridge if they're going to have to sit around for awhile.
Wild caught chucks will have parasite loads, they usually don't seem to cause a problem and some evidence suggests that some of the nematodes may even aid in digestion. If chucks are exposed to exotic species, they can pick up parasites they aren't equipped to deal with or the stress of captivity can result in a weakened immune response allowing parasites to get out of control. In these cases, intervention would be necessary.
I agree with Tom, give him sometime to fatten up on his own. Eve's chucks love bananna and if your guy goes for it, this could help him put on weight faster. Some chucks will also take insects, try waxworms since they have a high fat content. If he doen't take the waxworms, you can try mealworms and superworms. Offer the insects in moderation.
-Alice