Do you live in a fully selective area or is it an area with a few super selectives who admit regardless if area, on scores in rank order?
It makes a big difference to what the other schools are like in terms of ability range. In a fully selective area, the Sec Mods (and they are Sec Mods not Comorehensives because they don't have the full ability range) might be the lower 70% of ability based on what the test showed. In an area with few Grammars and where people travel miles to get a place, any non Grammar might still have 90%+ of the ability range - still not technically true Comps, but close to it.
I assume your area didn't have the Headteacher appeal system or you would have used it. Have you spoken to his school about if they have evidence to support an appeal - SATs predictions and the levels he achieved last year, other tests or classwork?
I guess you have to decide if you want to go through all this - you can, without even telling him, but it keeps the issue and the pain open for you, while it might be best to accept the situation and move forward to seeing the positives.
This will hurt a lot over the next week when allocations come out and people talk about it. If no-one else is going to a similar school from his school, you will feel different through the rest if the year - you just have to get into a positive mindset.
I wonder how much of your pain is about the other parents at the school? Saying you're going back to State seems to make you feel awkward and I guess that going to the non selective option makes it feel worse. You should recognise that you're probably hyper sensitive to others' reactions and even if most don't know much about the state options, they probably aren't despising you - absolutely don't apologise or suggest it is second choice or that you think it less good than their options. Always present it with a smile and the positives. Often people don't know what to say or how to react if you yourself seem disappointed with what you have got. And one other good thing, in an Independent, national offers day means far less - many won't have applied for State or even know the offers come out this week, so the big chat won't be about it....that phase may have already passed an independent school offers are already made.
When you get your offer, if there's a chance to go for another look, then do. Go determined to see the positives and to be positive for your DS. Even if you decide to appeal or go on waiting lists of others, it's good to work on the positives if the current situation for both yourself and son.
And do you know, as an aside, for all the threads about moving to Grammar areas, if this is a school in a fully selective area, I think this thread is a useful reminder for people considering those areas - what has happened to Op happens to very many people in those areas.