Brody the bully?

For as long as I can remember, Brody has been bullied. Kids at day care picked on him, cousin Evelyn overpowered him, the neighbors' kid called him a baby... the list goes on and on. He's submissive and he's sweet. He just takes the abuse from the other kids and doesn't retaliate, which makes him an easy target. Once, when Jacob was dropping Brody off at day care a slightly older kid (Nicholas) approached him and Brody instinctively backed into a corner. Whenever we asked about a new booboo received at day care, Brody always told us that "Nicholas did it", which we know wasn't always true.

Soon after Logan was born we moved Brody to a new daycare that has children ranging in ages from infant to 3.5 years old. Brody seems to have adjusted well. He plays well with most of the children and loves his new teacher, Miss Shahnaz. However, by combining children of all ages we've created a situation where there are kids smaller than Brody. Suddenly, Brody isn't bullied anymore, he's the bully! He has chosen 9 month old Kate to be the victim of his stored aggression. Jacob saw Brody shake Kate's highchair while she was eating breakfast and push her down when she approached him. The teacher told us that Brody gets along with everyone except Kate.

What happened to our sweet boy? Why is he picking on the little 9 month old girl who just learned to crawl? Is 2 years and 5 months too early for a kid to learn to pick on someone his own size? Or is Kate's 3 year old brother Alex going to have to seek revenge for his sister? Alex is huge so it would certainly teach Brody a lesson. Although Logan may be able to protect him.