C.C.
My daughter just turned 3 and she also has a VERY hard time with transitions. She absolutely loses it whenever any little thing changes - if she normally eats Cheerios for breakfast, but we make pancakes one morning, she melts down. So you can imagine how fun it was when she went from one preschool class into the next level up. Nightmare!
My daughter sounds a lot like your son, in that she also stops talking and starts making angry little noises when she is stressed out. I have to remind her all the time, "You have to use WORDS. I can't understand what (growling noise) means!" Or sometimes I will make the angry little noise back at her, but be even more exaggerated with it. Sometimes that will make her laugh, and then she'll say, "I want milk with my lunch and not water, mama!" When she does use words to tell me what she wants, tell her, "Thanks! Now I understand exactly what you need!"
One thing a friend suggested to me is Happy Thoughts. Whenever we're driving in the car or maybe right before naptime or whenever there is a quiet moment, I will ask, "What are your Happy Thoughts today?" Not only is it really cute to hear what made them happy, but it helps remind them of the positive things. I think when you're a little kid, life seems so out of your control, and it helps them to remember that fun and happy stuff happens all the time too. (As a side note, it really makes ME count my blessings multiple times a day as well. =)