I have never worked sleeves going down instead of up but the reason I don't want to join the sleeves and the body in the round is because of the shoulders. I have years ago made fair isle sweaters and the shoulders have that sloped off look. I think with the set in sleeve he will like the look better. I just thought of something.
If I did the body in the round until the shaping could I not do the sleeve in the round until the shaping? Then all I have to sew is the shoulders and around the arm hole. Would that work? Or would just the body in circ work.