Many reservists are public about what they do. I have my military affiliation in all my profiles. He's not in some kind of secret special forces unit or anything like that.
Ultimately, being a reserve officer is a matter of public record, as you are formally gazetted and there is no point trying to pretend it is secret.
Why would it be a poor idea? Unless he is actively a part of clandestine operations and units there is nothing materially damaging about this. No more than, say, folks like Chris Kyle, Dan Crenshaw, or Jocko Willink revealing they were SEALs.
> "The identities of the Twitter accounts which the 77th Brigade uses for engagement and communication remain unclear, however: the unit’s acknowledged account has been locked since it was hacked earlier this year, with the result that only approved followers can see its tweets."
Ultimately, being a reserve officer is a matter of public record, as you are formally gazetted and there is no point trying to pretend it is secret.