In the absence of any other mitigating factors that haven't been shared, the fact that he is already old for his grade would decide it for me.
I've got a child who is young for his grade, and the alternative placement would have been unbelievably inappropriate. His borderline birthday turned out to be a stroke of luck.
Obviously you need to deal with his reluctance, but my focus would be "How can I make him OK with this?" not "Should we do this?"
I like CAmoms suggestion of maybe waiting until the end of the year if you think the negatives would outwiegh the benefits of the immediate skip.
