M.,
1. Family
2. Hygiene
3. Diet
4. School/homework
5. Chores
6. Extra activities – friends, sports, music, movies, tv, computer access etc..
Hopefully you are not verbally comparing him to his sister. (That will put a wedge between brother and sister that could last a lifetime).
Take the cello and all extra activities away. He knows what you expect of him and he knows he has been able to “blow you off” and do whatever he wants to do without really serious consequences.
Time for the consequences, this child sounds like a smart boy. You and dad need to establish a schedule that includes a regular bed time, get up time (during the school week) and the chores he is to do on a daily basis. (He does not get the choice on what day he takes the trash out, or when he picks up his cloths, or what condition he leaves the bathroom in).
I believe a large mistake parents make is giving little children all these choices that they are NOT equipped to make. They start very young, “honey do you want this or that?” By the time a kid is 10 (and should be old enough to make a choice), they are clear that if they make a choice they don’t want to follow through with, it’s no big deal.
You have come to the point where it is a BIG deal. So sit your son down, show him the list and consequences and you will see a different child in about two to four weeks…..IF you and dad follow through.
Blessings….