Some of going back to a traditional school situation depends on the child's patience with retreading familiar material, too. An impatient, easily bored child isn't going to fare well with returning to a lower grade level. But age and maturity help with that somewhat, especially if there are challenges in at least some areas.
One of my goals with homeschooling DS7 is to "go wide" and not just fast. There's a limit to that, of course, but the independent nature of HSing allows us to study some subjects not usually taught in primary OR secondary schools. Archaeology, for example. Or code-breaking. This lets me challenge him without pushing him further out of synch with his age group.
As for re-entry and proving where he is: standardized tests help. AP exams, the Iowa Test of Basic Skills (or the similar California test whose name escapes me right now...), etc. can help to establish that your child has mastered a particular grade level. You might also ask to have your child take the midterms and finals for the grade before the one you want him in. If he can pass with a previously agreed-upon score (I'd aim for the 80%-ish range, but other people might shoot higher or lower. What you push for is your call, I think...), then he has passed that grade.
If you go with that latter suggestion, then I'd also do my best to make arrangements about what to do if he gets most things right but has one or two areas of deficiency. To cite a recent case mentioned on this forum (I think), a child who has never seen a minus sign might do all the subtraction problems on a test as addition. That doesn't mean he is incapable of doing subtraction. If something like that is all he misses, and it's from lack of exposure, not lack of understanding, then you might want to have an agreement in place about catching him up if only one or two areas are a problem.
