Fox is recessive, so both of the parents of the buck you have are both carrying fox. If you dont have any other foxes you could put him back to his mother and you should get about half of the litter being foxes and the rest being carriers.
edit to add: You also have to have the tan gene or the fox doesnt show, both parents must be tans as its dominant, although i guess if its a variegated it might not show. Potentially the parents are heterozygous tans so the fox buck may well also be heterozygous meaning he only has one tan gene (but because its dominant the tan gene has an effect). This means that the resulting litter between him and his mother could have some self babies although these too will have the fox gene too.