I too am unhappy about Rainforest Math being down. I never used it with DS5, but we tried it with DS4 a few weeks ago and he had great time.
Can HeyMath! be used as a curriculum? Hmm, I don't know. It does teach lessons (and very well), it includes practice problems. I would say it does a great job, but I would use it without any other material. It doesn't provide any tracking and you are free to explore lessons in any order.
On the other hand I think it's more than just a supplement. I really like it and I think it's the right choice for us at this time (DS5 works at the 4th grade level math). It's worth looking into. They have 15 day free trial.
Kriston, your son may like HeyMath! as well. They have lots of geometry and you can jump from one lesson to another. I am becoming a big fan of it.
