it does sound like it is fear aggression. The thing about this situation is, you have a relatively large dog who is barking and intimidating so in order to socialize him properly and get him past this, you'd have to find several men who are okay being used as test dummies so to speak. I would suggest getting involved with a private trainer who has knowledge about GSDs (and likes them) that can work with you on this. They can observe and offer suggestions as to how to handle the situations as well as see how YOU handle the situations. It could also be possible its time to pull him from daycare but that would be between you and the daycare. He could very well be intimidated and feel threatened by this one guy.
Perhaps with the guy he reacts this way too, it would be beneficial to schedule a sit down with the wife and husband and ask them to work with you on getting your guy okay with the husband. This is also where a trainer would benefit in helping.
Shasta GSD 5/5/10 CGC, ITD, TC
"Dax" Thor z vom Weberhaus BCAT CAX2 CGCA ETD HCT NCO-1 PKD-T RATI RATN RA TC TKA 3/18/2013
"Hades" Guilty vom Blutfeuer 10/26/2018
Zena GSD 6/1/03-2/16/2016
Riley GSD/BC 1/10/05-2/1/2013