Hi Bertram
first off, you haven't failed at anything, i'd take a guess that 90% of the parents reading your OP have done the same, and at least 5% of the remainder have short-term memory lapses 
it's so difficult in the early stages of assessment because you really don't know what you're dealing with, so please don't give yourself a hard time, are you able to talk to DS about what happened?
assure him that you are trying to understand why he feels the need to kick off the way he did, and that you only reacted the way you did as you were feeling his frustration too?
DD is 5, and has ASD. it was her monumental tantrums that lead us to the GPs, who sent us on our way to DX. at that point I couldn't even contemplate being in the house alone with her, i was not coping at all.
now i'm learning that once she's in that zone, there is a point of no return. i have to learn how to recognise when she's on her way there, and stop it.
for example, a busy day, one where she's worked really hard in school or we've been out into a full and busy environment (shopping, visiting family etc) are always tricky. she starts to ask when we're getting home, are we walking/driving/getting a cab/talking about what she wants to do when she's home, that sort of thing.
if we miss the cue, if she goes into meltdown, then the best thing we do is leave her to it. make sure she's as safe as possible (and hopefully out of reach of too many missiles) and let her stamp and scream it out.
in our house, it's the stairs - plenty of noise can be made by stamping and shouting up and down.
talk to DS about his meltdown when he's calm, let him know that the behaviour is not acceptable but you understand that if he's feeling overwhelmed and like he's going to 'go', help him work out what will make him feel more calm. DD loves torches and light, after we had visitors the other night, she was heading for meltdown when they left, so we turned all the lights off in the living room and let her play with that.
do you keep a diary at all? this is really useful when you're trying to work out your DSs triggers, not all triggers are what you might imagine (DD cowers in horror at hand driers, but will spend hours listening to light aircraft engines
)
lawks, i've written loads here, hope some of it helps