I've kind of always gone with the subjective, "can't get needs even close to met without the skip," as the test for skipping. Of course, we also went for having the school fill out the Iowa Acceleration Scale ;-). Have you had the IAS filled out for your ds yet and do you have IQ scores?
I really do think that early elementary is a very hard place for gifted kids as the options for acceleration are so limited. My dd skipped right at the end of elementary so I was less worried about long-term and whether she would still need the skip into middle school and high school as she was skipping to start middle school early and definitely needed it at that point (and still).
I guess that, in your case, b/c you're so early in elementary, I'd want a few things:
1) definitely the IAS showing him to be an excellent candidate
2) recent IQ scores over a group test (I haven't looked at your prior posts, so I don't know what type of testing you have as of yet)
3) some idea as to what the GT/accelerated programming looks like in your district and schools as he gets further into school and your best guess as to whether the type of programming they offer might serve his needs at all when he reaches that point
I might also talk to the GT coordinator if you have one who is reasonably familiar with your ds and get his/her feel for how different your ds is from other GT students s/he has seen over the years. In hindsight, even back to earlier elementary, one clue that we had that dd was not going to get her needs met in GT pull out programs, etc. was that pretty much every GT teacher and coordinator who had her in class was telling me that she was one of the most gifted kids they had ever taught and some of these people had taught hundreds of kids or more over the years. If you're getting feedback like that that makes it clear that your ds is never going to be the "typical" GT student in your district, a skip very well may be warranted.
