I went thru this many many years ago except my son talked me to death and would get into things and say the dog told him to do it. It is trying to say the least. It just takes patience and a little time. Maybe when your daughter comes home all three of you can have some fun time together for something special. When my youngest son started school my daughter would be all excited when he came home so we work on his and "her" homework together. It didn't take long before I had to send her out of the room when he did homework because she caught on so fast she'd give him the answers.
You might even try a little "home schooling" while the one is at school; pictures to color, etc so that the youngest will feel he's also doing school.
I wish there was one great answer but it may take trying a few different things to find one that works with your son. It'll get better and I wish I could give you a time frame but like everything else in life all kids are different. Good luck