D.S.
Hello C.:
When my daughter was born, my son was 2 1/2 years old. I tried to keep him involved/engaged in the things I was doing with the baby. I also, when I could, let the baby cry for a few minutes if we were in the middle of something. I would tell him that his needs were important too and that the baby could wait a few minutes until we were done. It showed him that he was important to me and my daughter was fine waiting a few minutes. Eventhough my son and I may not have been all the way done, we at least got to a stopping point before I got up. I always invited him to "help" me with her and walk with me to get her.
Also, I engaged my husband to become a part of my daughter's (the baby's) bedtime routine. It was a bonding time for the two of them and I then had time to reconnect with my son. Eat dinner with him or get him ready for bed and/or cuddle/read whatever time we were at that particular day. Then I would take the baby back if needed so my husband could tuck them in, etc.
Talking to his teacher/counselor/professional could be helpful too since there may be issues outside the home that are causing this negative reaction at home.
Hope that helps.