Thanks for all the great replies everyone. I think my wife and I (after many discussions with our son) have decided a while ago that we feel completing the skip is best, but on our first attempt we allowed ourselves to be talked out of it by the administration at the school. We plan to be a bit more firm with our ideas this time around barring any specific information related to our son that we have not been made aware of previously (I don't think any will come up). The things that have been mentioned previously mainly revolve around "socially awkward scenarios in high school" that might result from his skipping now, but we feel like holding him back from where he is asking to go (all of this was initiated by DS, not us) for 3-4 years in order to avoid a hypothetical situation that may or may not occur in 5+ years is less than ideal. We have a very tight family unit and would deal with any such issues as needed.
Size wise, he just had a check up and is roughly 75th percentile both height and weight for his age. He used to seem small compared to some of his classmates but is fitting closer now after some growth in the last year. Some of the higher grade students are still quite a bit taller than him, but he is 9 and some of the 5th graders are 11 already so there is quite an age difference in some cases.
I really appreciate all the great info in this forum. Thank you all so much!