I vote for offering the trial run as well. I think it's a little less dramatic when done mid-year, and I'd tell him it wouldn't be advertised as an "official" full-skip until after the trial period. The trial also makes the move very easy to unwind, if necessary.
Our son skipped from 4th to 5th mid-year with a trial period set for 60 days. His day was split with about 60% of the day being in 5th, sometimes more.
Thankfully, the receiving teacher said that he was satisfied with making the full & complete skip after only a couple of weeks, and the principal approved the change. I was happy because it turned out that DS was missing out on quite a bit when schedules didn't mesh perfectly. For example, he often found himself sitting for a test for which he had missed the preparatory class time.
Hopefully, your son will be more forthcoming as to his reasons and/or he will be agreeable to the trial period. The IAS is pretty firm on the issue of the child not being receptive.
Please do keep us posted.
