For another kid with a midyear skip, the sending class made a scrapbook for her and she sat in front of the class and received it and was "special" while the teacher explained to the class where she was going. They all said good-bye.
Good point. If you do choose to skip mid-year, ask the teachers not to say anything about it - just fade out before the break, and 'This is your New Classmate George' when he enters the new classroom.
Given that this will be a gifted classroom, there is a good chance that the social interaction will be better than before the skip. If possible, set up some playdates with boys from the class he'll be entering now, and then see how fast your son changes his mind about wanting to go to school with his new friends. Once your son is convinced, then DH will be easier to persuade.
Smiles,
Grinity
