Preschool Jitters

John starts preschool tomorrow and I'm nervous.
I'm not worried that he's going to cry when I leave, or that he's not going to have fun, or that he's going to love it so much he's not going to want to come home.
I'm worried that he's going to realize that he can't communicate with the other kids and it's going to damage his self esteem.
Developmentally he's a three year old, his temperament is that of a 3 year old, he's potty trained, he plays like a three year old, he knows his colors, understands counting, understands letters and that some combination of them forms words, can do multiple step tasks, gets the whole: in, under, before, behind, around concepts. The problem is, his speech is comparable to an 18 month old.
He combines 4 & 5 "words" together like a 2 or 3 year old should but when he wants to say "Mom, there's a big dog outside" he will say "Mommy! Durr woof out." Or if he wants juice in his Cars cup with the straw you'd hear: "Zah Mommy drr Zzzzzz sip sip peas". If you ask him if he'd like juice or milk, he would respond with an "Ummmmmmmm, ummmm" and you'd see his little brain trying it's best to say MILK!! But all he can manage is "Ummm" and then he signs "milk" or goes to the fridge and shows me what he wants.
It breaks my heart, it really does. I understand him, but very few other people do. He's very good at using signs or showing people what he needs or wants, but I know...I KNOW it bothers him, his Um's and pauses and frustrated looks show it...it's just NOT there. He WANTS to talk to people, to tell them things. He wants to tell them ABOUT something and when he does, they either look at him with a blank stare and look at me with utter confusion OR they ignore him!
He's been on the list for speech therapy now since January! I haven't heard anything from them since I got the notice that he'd been put on the list and that there would be a long wait. I was really hoping he would have had his assessment by now so I'd know whether to be worried or not or at the very least have some idea of when I can expect someone to work with him.
I hope his communication issues don't cause him trouble tomorrow and in the weeks to come. I hope the other kids don't think he's a baby. I hope he makes friends and that he's not laughed at, that he has fun and he's not scared when he realizes he's not understood, that he's viewed by his peers as an equal and not thought of as stupid and especially that he works hard to be heard and not just ignored. Most of all I hope we've made the right decision and that putting him in preschool will help him, not damage his self worth.

No one told me parenting was easy, but I don't know if anyone ever told me it would be this hard.