It's possible that Chevez could have originally been Chavis, which is a very well known free black family in NC around that time and I do believe some went down to SC early on.

I did a bit of searching for "chavis" using soundex in SC census for 1790 and 1800 and in 1790 Abbeville County there is a Margaret Chavis (listed as white) with a male under age 16 and then in 1800 Abbeville County (still listed as white) with 1 male age 16-24.

1800 census, as you know Alexander Cheves has a male 16-24 and 1 male over 45. Anes Chavos in Barnwell, Barnwell Co. has 5 FPOC. Hannah Chavos in Barnwell, Barnwell Co. has 10 FPOC. William Chevas of Greenville, Greenville Co. has 6 FPOC. He really could be in any of of those household.

Possible spelling variations to use when looking for him: Chevez, Cheves, Cheeves, Chevas, Cheevas, Chaves, Chavez, Chavis, Chaviz, Chevers, Cheevers, etc. And these are all variations I've seen for the Chavis surname.