I don't have quite the same issue but I have looked ahead and made decisions based on the distant future. I have less of a problem because DS is not a math prodigy and is also equally strong verbally. We chose to wait to do Algebra next year (5th grade) even though DS appeared ready by every measure. This way he won't start Pre-Calcuus until 8th grade, which will leave him enough math in high school - Calculus, Differential Equations, Linear Algebra and Statistics (current offerings in our district). There is also a lot of math horizontally. DS has picked up odds and ends by reading interesting math books (not textbooks). DS has expressed some interest in business math and econometrics. My thought is to help him develop an interedisciplinary base, which is actually more beneficial in the long run. Of couse, he won't be ready for something like econometrics until he has mastered Calculus and Statistics. He will also likely do some competition math.
You're lucky to have those courses in your district. Our school stops at Calculus (and it has Statistics, but not Differential Equations, Linear Algebra, or anything else). So our son was going to run out of courses no matter what, and we'd be having to find things elsewhere anyway, so there was no point holding back.
I agree with developing an interedisciplinary base, though I'm not sure how to do it.
