Hi and welcome.
My son has a 121 at Mainstream and in the very rare times he has been sent home his 121 is actually there with him.
Personally I would do as I did - if you can that is - and get involved in the school on a day 2 day basis but not with your son's class - there is always something to do and then you can see for yourself.
Also I would ask to speak to his 121 when you collect as you "have been asked by his paed to collect a detailed description of the events" (regardless of if you have or haven't).
I agree you need to tread carefully but I would want to get to the bottom of this - including hanging about at going in time and seeing his 121 in situ.
Sorry - sound very devious but I think that you need to do your own snooping so that you have your facts - without asking for the personal sign in sheets (or something that proves the 121 is there) it will be difficult - also you will be able to "spy" and see what is going on in your son's class - i.e. is his 121 deployed elsewhere.
Also, ask for their SN policy on handling of staff changes. Just ask so that you know you can prepare your child and also understand what happens and how the supply knows about your son. TBH a class teacher absence or supply teacher in should not change the 121 issues in the class.
Lastly, the next devious bit - get to know some of the louder kids in the class as their parents and get them to talk to you - it is surprising how much you can learn from other kids!!!!!
Hope this helps a little - even if it is devious

and do talk to the school - your SENCo should help and also you should have a statment review meeting soon - call one sooner if you need the Ed Psych or someone else independent there to help.