What do you think triggers the tantrums , what happens prior to them happening, at this age you can start to have time outs first I would see if you can figure out why is is having them as something is upsetting him, does he get onwell with the 6 year old, is she kind to her younger brother ? Is it possible he could have heard a lot of talk about a new baby and is feeling left out and jealous,It may be a good idea to focus on his positive side and praise him when you seeing doing something right ..