Change of circumstance

I currently have joint custody of my son (not 50/50)

I have my son from Friday night until Wednesday morning every other week (I never have him on Thurs night / Friday morning)

My son is having trouble in school and is doing poorly on his Friday (end of week) tests

His mother openly admits they do not always study for the Friday test and blames our son for not bringing the materials home. She has not suggested any resolution just uses this as an excuse.

Recently I had my son on a Thursday night (only time ever) and he scored %100 on his Friday test. I am aware this was a single case but it is a fact and the only time I have been able to have him the night before his tests.

The school is suggesting he is not paying attention in class (I dont believe they would point to a parent) My x is running with this and the only suggestion she seems to have is taking him to the doctor (leaning toward ADD etc). I am strongly against this when I know we could be doing more at home and I believe parents need to take more ownership when it comes to helping a child at home as well and installing proper goals and study habits that will effect his attitude in school.

I believe if I had my son the for entire week rather than the first few days, I would make sure he brings his materials home and I believe he would do better on his Friday test. If he was doing better on his Friday tests the paying attention in class part would by default be less on an issue. I also believe showing him proper study habits and setting goals at home would carry over to his school day perhaps addressing the paying attention in class as well.

My question is do I have a case. Given what I have said could I argue a change in circumstance and ask for the extra two days (Wednesday and Thursday night).

I have my son from Friday night until Wednesday morning (I never have him on Thurs night / Friday morning)

Every other week, saw this after I entered it.

At this point in my opinion it would be hard to prove there has been a substantial change in circumstances which warrants a change in custody.