IB schools, as a group, tend to be great for very 'not 2e' gifties who are fine with lots of product, organized, ok with being given lots of time consuming projects. If your son is organized, attentive, and has above age group emotional maturity than it's totally not an issue. If not, keep an eye out about how these things play out at your local school.
I wouldn't be worried about joining in at 5th, as my son joined a school in 8th and I was so worried, but it turned out just fine. He said he had 'new kid' appeal. Apparently one can't 'time the market' socially. A better question is 'what is the overall character of the kids in this cohort.' Each year going through has a bit of a texture, and it's great if you hit it lucky enough to be in a group with lots of kids who have good attitude towards school.
Anyway...seeing the classrooms in action is basically the only way to know what you've got to choose from. Good luck!
grinity
