Awwww...I'm laughing and feeling your pain at the same time. They're such cute little irrational messes at that age, aren't they? Now that he's a bit older, maybe he'll be ready to see you sitting the audience and will hold it together. Also is it possible for him to spend some time with you in your seat before they have to go back stage and get ready? Maybe that will help calm his excitement and emotions a bit. That is really just too sweet though, isn't it?
FWIW, my oldest (now 14) didn't participate AT ALL in his little daycare/pre-school assemblies when he was 2 and 3. He'd practice, get dressed up, we'd go, and then he would refuse to even go back stage or would get back stage and refuse to come out LOL. When he was 4 he finally participated by going on the stage but then would stand there with his arms crossed, looking sulky and sad. It was pitiful!
Whatever happens, enjoy the event even if he falls apart - time flies, it really does!