Actually, it would be congressperson, not Congressperson. You only capitalize when you are referring to a specific person and their title, i.e. Congressman John Doe. And, while a Senator is a member of Congress and technically a congressman/person, you NEVER refer to them as such.
I interned for my congressman for a year and a half and spent 10 years lobbying members of Congress.
I'm actually shocked your "congressman's" office had you refer to him as such. I interned in a members office and am friends with lobbyists and we are all very cognizant of using "member of Congress" or "Representative", but never congressman (and I have literally never heard anyone use the term Congressperson). Representatives don't have an honorific, so they are just Mr or Mrs So-and-so, not Congressmen So-and-so or Congresswoman So-and-so when addressing them directly. (Unlike Senators, who you would call Senator So-and-so)