You can ask, but he might not pay. How it was *supposed* to go with my DH was she had joint legal and visitation, and was supposed to pay 50% of things like extra curriculars (which she approved prior to enrollment), school supplies, doctor copays/medical bills/orthodontia. What ended up happening is that she would buy school clothes and cry poor the rest of the year. DH did not force the issue of paying CS so she never did. Honestly, I don't think it's worth going back to court over this. The schools will learn who is the best one to call and who isn't. I'd focus on taking care of the child vs making his father toe the line or having sole legal just because he's not using it. Save your money.
What you can also do is go to the school and talk to them about the reality of the custody situation, and give them alternate people if the parents cannot be reached. DH wrote a letter for the kids' files every year so that if they needed someone and he wasn't available, they could call me or my ILs or a close friend of the family. (Their mom being 1.5 hrs away most of the time.) Don't be "that mom" just say, "I want you to be aware that my son is usually with me and you should call me first before trying to reach his father in the event of an emergency." Be professional and neutral about your ex.
Tell your ex about the events (even just a quick "here is the school calendar and Josh has a parent-teacher conference on x date. If you cannot attend, here is the teacher's email address for a different appointment") and then let it go. Be short. Be professional. What he does with the info is his problem. If he lets the kid down, let it be on him.
I went to most of SS's football and wrestling matches. I watched SD squeak her way through years of band and then watched both of them on stage for various theatre productions. I was there and that is what mattered to me. The ONE time we mixed up the venue and missed SD she was so upset - because we were there. Always. When her mom showed up, it was a treat. But she could always count on us. That is what your son will remember. So be the parent that is always there and let your ex define his own parenthood. Even if it's not what is ultimately best for his son.
If circumstances have changed, you can probably get a review of the CS, but I cannot speak from experience about it. If you ask him for 50% of bills, what does your CO say? Does it need to be submitted with receipts in writing within 30 days?
I won't say it wasn't maddening to receive a child who needed to see the doctor and wasn't taken in on his/her mother's time or to truck out to CVS on a Sunday night because somewhere in the mall crawl their mom couldn't buy crayons and glue -- but at the end of the day it was more important to do it than to get into a tit for tat with her. You cannot force him to be a better dad. Pick your battles.