I could see this as a potential freak out situation for the guy. Seeing the child...what if it looks like him..and the million random thoughts that go with that. Perhaps awkward conversations with your husband or other family members. Thoughts of starting his own family etc, etc, etc...the whole thing being some what surreal. Maybe.
Also, from a kids point of view, birthdays on or around the holidays suck , tend to get lumped in ...one cousin (dec 20) and my uncle (dec 27) had that .....so if it's an option to go early or hang on late....might be something to think about or investigate further, talk with folks who are in that boat.