I know it seems unfair but you have to look at it from an outside perspective. He has a relative in the U.S., which is a strong incentive for him to stay in the U.S. rather than go home. He is a young student without much income, which is not a strong tie to his home country. Additionally, your willingness to pay for/financially support him for this trip doesn't really help his case, because you could also financially support him if he overstayed. Further, tourist visa applications are reviewed under the lens of presuming the person has immigrant intent - if he doesn't have strong ties to his home country, he cannot overcome this presumption. Lastly, as others have noted, his home country is considered a high-fraud country, due to previous individuals overstaying and misusing their visas.
If/when he has a career, owns property, and/or has a family in his home country (spouse, kids), his odds will be better but still no guarantee.