I'd make sure he's not being bullied at school or abused by anyone in any way. The fact that the change is sudden makes me wonder about that.
I would also have the school test his IQ. And maybe look for a charter school or program to supplement this school--even if it's an after school program, it may be incentive to get through his day. Ask the teacher to give him some more challenging work. I don't know how schools work nowadays because we home school, but maybe the school has some suggestions. I'm sure your child isn't the only advanced child they've ever had.
Regarding the behaviors affecting your daughter, I'd always have a back up plan. Take him to the car and let dad and sister enjoy dinner in the restaurant without him. Have someone available to pick him up from an outing if he doesn't behave. Meet a friend at the park and have the friend look after your daughter while you sit in the car with your son until he is ready to behave. Make sure he doesn't think you are suffering by doing this--tell him you don't mind sitting with him until he is ready to have fun. Take a book and read. Tell him to calmly let you know when he's ready to apologize and behave well so he can rejoin the others.
Why don't you go to Barnes and Noble and get a McGraw Hill Spectrum book that might challenge him and ask his teacher to let him do pages in it once he finishes the work she gives him? You can work with him after school, but it really sounds like he needs to be challenged at school.
I do mean out to dinner at his favorite places and such and I have spoken with his teachers and counselors. They want to put him in an anger management group at school. His teacher says he does great except that when he gets bored or thinks an activity is "stupid" he will act out. He has an older sister who had troublie in Math so I always had math problems lying around the house. She is also an avid reader. Consequently he is ahead of others in his grade level academically. He gets bored and acts out horribly. There "gifted" (despise that word) program doesn't start until he's a bit older.I have asked for an older "buddy" for him to do extra homework sheets with. They sent someone twice and then that boy was too busy with his own school work and activities. He just gets so angry when he doesn't get his way or when things aren't going as he thinks they should.
Everytime I ask the question will it end soon I am told in about 20 years. I know I didn't think it was funny when it was told to me.
Is there an issue at school? Is he being picked on from someone? It seems like there is something bothering him, and he just hasn't told you. You might want to sit down and talk with him and ask him if there are any issues?
Which, I just read April2's entry and it is pretty much the same as mine, so it makes sense.
I'm thinking maybe she means "dinner out'?
Have you tried talking to his teacher and seeing if there's anything going on at school that you should be aware of? It's possible he's being bullied at school or maybe by the neighbor kids. There's something going on for him to have this much anger, especially if this is a fairly new attitude. He sounds to me like he might be stressed about something. If I were you, I'd talk to the teacher first and also watch what he's watching on TV and the people he is around to see if he's picking up on things you don't want him to pick up on. Let us know how things work out. Best wishes.
Special treats like dinner? What does that mean?