Hi futurenurse!

Congrates on finishing nursing school! I think your instincts are right, he is frustrated, and doesn't have the verbal ability to express it, so he screams. It is really quite normal. But please don't give in to that, give him what he wants, or he will learn that it is acceptable to do that whenever he doesn't get what he wants.
In fact he is very smart, he has learned the power of screaming! It sure gets everyone's attention. I know nursing students at my college are required to take a child development course. If you didn't, find a good book on child development to know the stages children go through. Your son sounds very normal to me. Now if he starts to do the tantrum, which it sounds like he has, just do the same thing. It is hard I know.

If this happens in a store while shopping, just leave the cart and leave with him. I know it sounds like a pain i if you are grocery shopping, but he will soon learn that screaming will not get what he wants.

Time out is good, and the reason he is good for 1 1/2 hour, then does it again is that he forgets. Children's memories are short at that age. Keep doing the time out. It is exhausting, I know but teaching them now when they are young is much easier than when they get older. His screaming will probably stop when his verbal skills become better. You are doing the right thing.
Please don't slap him, I cringed when I saw that as a suggestion. That will just teach him to hit when he is frustrated instead of screaming.
All abused children were only "spanked" or "gently slapped" in the beginning of the abuse, please don't anyone hit, slap their children. There are much better ways of teaching your children. Even the American Pediatrics have said spanking or hitting children is not recommended.
Being a parent is so hard, but it sounds like you are doing a good job, believe me, you work hard on them now, it will get much easier when they are older and you will be glad you did!. Good parenting is hard work but so worth it in the end.