Hi I have a few ideas i think might help you out.
First of all, I think you should talk to him about your mother's death. I think you need to make sure he is dealing with his emotions about it properly. I would recommend talking to him one-on-one, and gently asking him questions about how he feels about it, and try to really understand what's going on in his mind about it. Even though he's too young to really understand the concept of death, you should still make sure he isn't having any emotional issues with it that could really affect him in the future.
Second, I think you should try your best to include him with your pregnancy (if not already). Invite him to sit with you and feel your tummy for the baby's kicks, ask him to put his ear to your tummy to listen for his new baby brother or sister. (who knows what he will actually hear, but make it sound like it's a really special way to connect with the baby). Ask him to help when you go shopping for new things for the baby. Invite and encourage him to help with preparations for the new baby, such as getting the room ready. You could even make it his "special job" to make sure the new baby has a stuffed animal, toy, book, etc so he feels important, and will hopefully not feel jealous about the new baby.
As for his fits, personally, this is how I would deal with it: When he does something bad, for example if he hits you, give him ONE warning. Tell him in a firm (but not mean) voice "No hitting, or you're going on timeout". If he hits you again, bring him to timeout and tell him "you're on timeout for hitting after mommy told you no" & walk away. If he tries to leave timeout, silently put him back on timeout. Make sure you don't say anything to him, don't show him any anger, frustration, etc, just act serious and silent. Don't let him see you look at him while he's on timeout, make him believe you're ignoring him, and don't stay too too close to him. Once he stays in timeout for 3 minutes (one minute per year of age) go speak to him in a calm, gentle, but still serious voice. Make sure he tells you why he was in timeout, and make sure he apologizes. If he gives you attitude, that means he's not done yet, & tell him he has to be nice or he has to stay in timeout (still talking in a gentle, serious voice). Once he's done his timeout, and has apologized, give him a hug and/or kiss, and go together to start a new task that he will enjoy, such as playing a game, or reading a book.
Overall, PLEASE make sure you stay extremely consistent with him. Consistency is key to successful parenting because if you keep trying different discipline techniques with your child, he will just become confused and frustrated. If you stay consistent, he will know what to expect, and will understand what is going on. Hope this helps!