AoPS or other Pre-algebra for 6th grade??

Is anyone familiar with AoPS high school math (pre-algebra and above)?  I have heard that it encourages students to appreciate the concepts rather than just algorithms and to work with varied problems to find the answer instead of being told how to do them and practicing memorized algorithms.  My son is good at math, and I want him to continue to see the beauty in it and be challenged just enough but not overwhelmed.  Right now he is somewhat bored and frustrated that he doesn’t get the answer right when he flies through the “problems I already know how to do” and makes sloppy careless errors.   I know this road will lead to horrible habits!  He is starting to think of math as a chore to get out of the way when he has always enjoyed it :(.  He is going into 6th grade and has finished RightStart B-G, Singapore Challenging Word Problems through level 5, and most of Singapore 6b.  I was thinking pre-algebra or algebra next?? He is the oldest of my 3, so this is new territory for me. Any advice would be much appreciated!