Dear Ericka,
At your son's age, there's really one necessary ingredient in such circumstances, and that is to manage the behavior. Immediate time out at any hint of aggressive behavior is the sensible intervention, but it will also be important to regulate his interactions with other children. For example, he may be required to busy himself with an individual play activity and diminish the amount of interactional play.
Also, be alert to any unusual stressors. A rise in aggressive behavior can often signal that a child is under some pressure in his environment. If that is the case, of course, you'll want to lessen or eliminate that stressor.
Diet can be a big influence on behavior. Try keeping a log in Excel and log everything he eats and his behavior every day. Keep in mind that it can take two or three days for something to affect behavior. After a couple of months you should see a pattern. Kyle tested negative for dairy allergy, but after I started logging his foods I noticed how often he ate yogurt and cheese and when he was acting up. After only two days of no dairy his behavior improved. Kyle also reacts to artificial dyes and anything with corn syrup. I also limit sugar and 99% of junk foods. Cook from scratch as much as you can.
Try adding essential fatty acids to his diet. When Kyle was four he began hitting, scratching, spitting, etc. I added essential fatty acids and after only two weeks his tantrum stopped. Try reading "The LCP Solution" by Jacqueline Stordy and Malcolm Nicholl.
If he only acts up at the babysitters it could be the stress of peer relationships (one of Kyle's big problems. He has very poor peer social skills and therapy helps with this. We had to eliminate our babysitter because he started hitting the kids there. My husband is on disability and can keep Kyle after school. I worry that Kyle does not have anyone to play with and interact but for right now he is happy as a lark since his major stressor has been eliminated. I hope maturity will improve this problem.