Try giving him a supplement ask your local health food store which one for his age, it has been shown that a multiple vitamin antioxidant supplement has the same efficacy as Ritalin , prescribed for ADHD and realise he is feeling hurt he is not the Babyany more , give him lots of positive attention and one to one time , make sure he still knows he is special , this is very common and most children behave the same if not worse, ask his Dad to play more Physical games with him,and let Dad have the Baby whilst you read him stories and have some fun with just him Good Luck
Do you get out to play somewhere physical outside the house everyday? To me, that was critical with preschool boys. If you go to the park, the animal farm, McDonalds playland, gymboree, etc., every day that really sets the tone for a calmer day.
When I had two preschoolers at home, we'd get up, eat, hang around for awhile until about 10 and we'd be out the door for a couple hours - with granola and juice boxes in the car. Come home, have lunch, play at home, nap, fix dinner, etc.
Getting out every morning made a huge difference.