Yes, you can just add him. My daughter has done this several times. I just went to the front desk and told them she was coming for a few days, and I wanted to notify them. You have to give them her name so they know to let her in at the gate (oops Him, I mean). Otherwise, they will not let anyone in who is not listed. They can charge you for those days. However, the CM's I informed of this said they would put her down as a "day guest" and she would not be charged. So, never had any charges done. As far as tickets/adr's, we just added her name to our my magic reservations as a guest, and made her reservations the same as ours. No problem.
I think Disney likes to be informed of extra guests and are grateful when you inform them. I imagine some people just don't do that thinking they won't get charged. We found it to be the other way. We have never had a problem with this, just inform the desk of his name and when he is coming. I would add that he is active military too. Have a great time with him