Pouffe
Feel for your son. School can be very tough indeed for many children regardless of where they are on the autistic spectrum. My son has a Statement and am bloody glad that he does (it is only because of the Statement being in place that teachers actually realise he is not being deliberately obtuse/difficult with regards to his spoken word).
Your DS does not need behaviour support (what do these teams exactly do, short answer nowt!) so much as a Statement. Having a legally binding document like this in place may go some way to making his school life easier as his additional educational and social needs would be properly documented (and thus the school would need to take notice).
Did the EP observe your DS in school or did you see such a person privately?. Did EP make a recommendation for statementing?.
I would apply for a Statement now regardless as this can take six months to set up. Would not worry about a potential fall out with school, what they have done to date has not really helped. Lunchtime support is not readily obtainable without a Statement being in place and even with one some LEAs are loathe to provide such cover.
As for Senco saying you don't have enough evidence - tsk to this person.
Have a look at IPSEA's website as they can be helpful and there are model letters you can use for the LEA. You would need to write to the Chief Education Officer at your LEA and give them six weeks to reply.
SOS:SEN can also be helpful and could the NAS.
Re the meeting on Monday do not use the words, "may have".
You are your child's best - and only - advocate. Never forget that, you are truly best placed to fight his corner for him because no-one else will.