Middle school has many school based math competitions such as Mathcounts, CML, Math Olympiad, AMC8. If your son really has passion in math, he would love it. IMHO, worksheets are only useful for K-5. Your son definitely want to spending more time on mindcraft over the boring worksheets.

Someone already mentioned the importance of problem solving. Art of Problem Solving (AoPS) is actually the website and kid-friendly online community I would recommend for your son.