I have another comment re: subject versus full-grade acceleration. When DS10 was in elementary, he did not like to stand out, which also meant he did not like being pulled out for other classes. For him, a full-grade skip was preferable, as he stood out less. Subject acceleration would be great if you have an entire school set up to do this -- main subjects are taught at the same times, all kids go to their levels. But that is rare. The logistics of subject acceleration are generally trickier.
TripleB, from all you have said, I would say you have nothing to worry about re: the skip. Sounds like he would be moving to an amazing teacher, and if she administered the test and believes he may still be underchallenged, I would believe her. That is the risk with highly gifted kids in regularly paced school programs. But if she thinks he might be underchallenged even with a skip, imagine what he's learning now.
Go ahead and make all your lists of pros and cons. Seem there are fewer cons based on the info you've shared, but obviously I am a stranger on the internet.
You know your kid.
My son is also not especially sporty, and he didn't do quite as well as the other (older) kids post-skip, but it is doubtful that he would have done very well with agemates. Now in 6th, he can hold his own.
I always felt my DS was stronger in Language Arts than Math, but he does equally well in both (and when we first started asking for differentiation, we used math as it's easier to show the advanced level there). It's hard to predict at a young age what their strengths and interests will be. Sounds like your kiddo does well across the board, which nicely supports a full-grade skip option.
