Don't know about Mathemporium, but I know lots of kids who are very good at math and go to Kumon. It's self-paced , so some kids get material way above their grade level at school.
Don't know how old your son is, but if he is 4th grade or older, you can also look at math competitions and talent searches. There is also a thing called Math Circle that is for very advanced kids, you can google and see if there is one near you.
Or you can organize a math club. I just did at my son's school. It's working really well and I'm just so happy to see that so many kids and parents are enthusiastic about it.
