The simple answer? He most likely needs to be dewormed. It is very possible that he has picked up an intestinal parasite on his ventures. You can contact your vet for a dewormer or pick up a product from your local pet store. These are usually not as good though and I would stick with a product from the vet that covers all intestinal parasites like drontal plus.
Other concerns would be whether his is getting into fights with other cats and if he has picked up a nasty virus like feline leukemia, feline immunodeficiency virus, or FIP. In these instances I would have testing performed to be sure that these are not present should he not improve after a deworming.
If he is going to out running around then I would be sure that he is on a high quality diet to help sustain him.
Let me know if further assistance is needed.
Good job & thanks'