Check his mouth and see if he has bright pink gums, and then watch him when he goes to the bathroom, to see what his stool looks like, a good sign is if it is too loose, if he has a firm stool, thats a good sign, and note how long has he been having this problem. You can go to any feed store and get worm medication and try that, Usually if a dog doesn't eat it's because of some kind of digestive or stomach issue. Also if you can't afford to take him to the vet there are clinics all over the country now that have weekend specials just for this type thing, At Tractor Supply a lot of the time. Hope he gets better soon