Yes. Define the household rules and place him in a timeout per misbehavior. Your post mentions nothing of discipline. This is what your son needs. If you need help with discipline, let us know exactly what you are currently doing so that we could make appropriate recommendations.
Your problem is really that you had another child when your first was old enough to know he was your one and only, and young enough that his emotions are still primitive. It's like the opposite of the "sweet spot." How happy would you be about it if your husband moved a beautiful woman into your house and told her that you would have to share not only him, but all of your personal stuff with her, and calls you "mean" when you are upset? I would bet you would sooner or later throw a tantrum too. And you are an adult, whose emotions are not primitive any more.
Your best bet is to avoid the triggers. Why does he have to share his toys with his brother? He already has to share you, who he once had all to himself. The toys are just a tangible symbol of all he has lost when the little usurper was born.
Please give him the integrity of his own stuff, his own space (even if it just means gating off a part of a room) and one-on-one time from you without his little sib present. He's hurting, and it will help.