Hi, glad your here. Beleive me we have all been there before, after years of keeping and breeding collareds I still take in emachiated, sick or injured collareds people send me 
Anyway first thing to do would be to check him for parasites/worms, sounds like he was a wild caught lizard and they usually have them. Probably be treated with panacur. Then you could begin to get him fattened up by offering him crickets, superworms, waxworms, grasshoppers, I think he would fatten up after awhile.
Also are you SURE it's a male? Maybe it is a female who just layed a clutch of eggs, she would be extremely thin also. So check sex if your not sure. I have BIG photos on my site that shows the differnece. Or post a pic and we can help you out.
Suncharmers/ Collareds/ Care and More
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PHEve / Eve