It would help to clarify the skip. Is the plan to skip him immediately (+2), or to finish this year in 4th and promote to 6th for next year (+1)?
My DD9 did a +1 skip by unenrolling from public school in 2nd grade, immediately beginning homeschooling as a 3rd grader, and re-entering public school as a 4th grader the following year. It has been a big success.
The best way to identify how your child measures in maturity versus age norms is to observe them interacting with age peers, and with older children. Getting your child's feedback is also a big help.
Some areas we found worth considering for measuring maturity:
- Thoughts - Does your child get frustrated that they often have to explain themselves to age peers?
- Behavior - Does your child exhibit superior impulse control, and become outraged at the impulse control of normal age peers?
- Physical - How does your child stack up in size/mass compared to age peers? Can your child contribute in a team sport with older children, or would your child be the last pick?