Which online Math Programs would you recommend for an 8 year old who is in 4th grade?

My son was moved from 2nd to 3rd grade in the middle of last year. I was afraid he missed many of the basics covered in the last half of 2nd grade and the first half of 3rd grade (the two half years he skipped) but he didn't have a problem with the 3rd grade End of Grade Test.

I think he's currently bored with what they are covering in Math so I would like to get him working Math problems that are challenging him without skipping over the basics entirely.

I'm trying to give him challenging problems myself but I think he would also like to be working at his own pace and covering various topics through a computer or online program. Something that tracks his progress so I can make sure he is understanding the new material would be fantastic.

Thanks for any recommendations!

TripleB