IME NJ districts are very hit or miss when it comes to G&T. My district, for example has NO G&T program. We also have 1/2 day K, and I would say there's essentially no math instruction.
If you want to look into online courses, check out the JHU CTY program: http://cty.jhu.edu/
They have self-paced mathematics courses that could be enough to supplement what your son is learning in school.
Another fun enrichment option is learning programming. Scratch is fun and pretty easy for kids to use: http://scratch.mit.edu/
My brother and friends did the enrichment program at Montclair State during elementary. They did it in the later grades. Fwiw, those kids ended up going to Princeton, MIT and Pomona.