IXL can be brutal and I know of many kids brought to tears by their grading algorithm...but my son still thinks it was worthwhile.
Initially, he would keep going until he got 100 on every exercise but finally he would go until he reached 90, then keep going but if he missed one he would stop and consider the exercise complete.
That is wise of your son to look at it like that. It comes in time I guess.
Perfectionism made it not fun for my son when he was younger I suppose.
He got better with Mathcounts Trainer and Alcumus, He was still to worried about being on leader boards than trying to complete to 46 and moving on.