Aww firslty giant hun, it must be so very tiring.
A couple of questions for you:

1. Have you had him checked by a Dr
2. Do you live alone or with a current partner etc
3. When you say we will leave do you pick him up and leave?

I agree with krysalyss, sugar is not always the cause of these kinds of things but your doing a wonderful job trying to find the cause, some people would just say ahh thats him and not do anything.

I guess i would suggest maybe seeing a Dr or a paed, maybe asking the local MCHN if there is someone you could see.

Also if you make a threat about leaving if he doesn't behave then get up and leave, apologise to the people and say we must go.
If possible take him places he likes, tell him he needs to behave or we will leave, the minute he misbehaves you grab him and take him home. Once he realises he's not getting away with it and even the places he likes to go he might start to realise you mean business.

You may even find it could be something sensory, my friends son has sensory issues, he is easily overloaded and can't always control himself, there is a thread around here somewhere with some info, maybe check with some of the girls there and see if their children have similar issues.
I know my friends son has issues with speach in the matter of repeating things and talking like a baby!

GL hun, i've prob been no help at all, hope you get some answers.