If an RFA does not sign for a season, they do not accumulate a season towards UFA eligibility.  A player needs 7 seasons or be age 27 to be UFA eligible, and so that missed season would not count towards the 7 required season.  Since the player is an RFA, that means the team issued him a qualifying offer, and they would therefore keep his RFA rights for the next off-season. The only exceptions would be if based on his age and career experience, he's reached UFA eligibility (Group 3 or Group 6). 

Information on UFA eligibility: https://puckpedia.com/salary-cap/unrestricted-free-agents-ufa