A: When you think about it, it's pretty easy to understand how a drooling, stinky-poop-making, crying, hair-pulling, waking-at-all-hours-of-the-night individual would not be the easiest thing for an older sibling to adjust to. But there are ways to help a child learn to get along well with a sibling.
Teach and praise his being a "Great Big Brother". Parents can help a child see himself, as well as his relationship with his sibling, in a different light. Catch him when he is being good to his sibling, such as when he helps the sibling do something or talks nice to her, and praise, praise, praise. Keep using the phrase "What a great big brother you are" when he does these things. Brag to others (in his presence) about when he does these things. Talk to him about how proud it makes you feel that he takes such good care of his sibling.
Make sure to spend individual, special time with him. With a baby, life can get hectic, and it is easy for an older sibling to feel like he is not getting enough attention. Also, since a baby requires a lot more of mom's time and attention, it can feel to an older sibling like he is not as important or loved as much, and resentment can follow. Make sure to set aside special time just for the older sibling and each parent. Make this time in which something fun and warm and loving is done, as opposed to time in which he is being told what to do or is being corrected or questioned about events in his day.
Find what he does well, and help him to pursue this. Everyone needs to feel special and accomplished, and finding what he does well in life can help with this. Whether it is art, music, sports, academics, or gardening, if a child finds his special talent and is allowed to nurture this, it can make a big difference in his life.
Keep in mind that not all problems may be related to a sibling. If a child is having behavior problems, it is possible that this is related to the presence of a new baby, but it is also possible that it is not. So if, for example, a child is having problems at school, it is important to get as much information as possible about the child's difficulties by talking with the teacher and possibly the school counselor, and then taking whatever steps are appropriate (for example, working with the school on appropriate interventions or possibly setting an appointment with a child psychologist).