So, I do understand your concern but please try and reframe your understanding of the assessment in a different way to failure.
He has difficulties that many of his peers don't right now and it is right that you look at these and see what needs to be done to support him to progress. However this really isn't failure.
You need to know that child development isn't a linear process, as if a child is on some kind of conveyor belt of learning and development and that if your child is behind his peers on that conveyor belt at one point he will remain so for life.
Child growth and development is much more up and down. Just like physically, children can have growth spurts at different times, some start losing baby teeth at 5 and others at 7, some enter puberty at different ages to others, intellectual and emotional development also varies too.
The absolute brilliant thing is that school have identified that he needs additional help so quickly and are looking to provide it. They will also advise and support you on how you can best help your son.
It's totally understandable that you feel anxious and upset that this has been flagged up, but the aim isn't to label your son as "failed" but to identify the right support to help him to catch up.
It may be that with support your son may move forward in leaps and bounds and catches up quickly with his peers' level of understanding. It may be that this is the start of a much longer journey where he needs additional or different support for much longer. It maybe somewhere in between these. I don't think it is possible to say right now, but please don't jump to a worst case scenario. If you work in an SEN environment it may be easy to assume that it it will mean more serious lifelong difficulties as that is what you are surrounded with day to day.
It may help to realise that actually your son hasn't changed at all. He is still the same lovable, active delightful ( and probably sometimes infuriating) little boy that you have always loved and cared for.
If you can, be pleased that the school are so willing to help. Ask them the best things that you can do at home to help and then be consistent in making time for those activities as much as you possibly can. Work in partnership with the school to do the best for your boy ( I'm sure you know this and are very willing to do this anyway.)
So, for now this might look like saying to his class teacher. "I know you are going to take more advice but in the meantime, what do you think are the most important things I can do to support him.". Then just take it from there.
I don't know your son and it may be that these suggestions are not appropriate but you might try some why and because printables from Twinkle
https://www.twinkl.co.uk/resource/t-sc-200-autumn-why-and-because-scene-and-question-cards
Also use whatever toys he enjoys playing with and play with him but narrate whatever is happening or give simple instructions or opportunities for speech (eg " Oh no Thomas is going into the tunnel. It is very dark!" " Now he's going up the hill. The carriages are very heavy. Will he make it to the top? Does another engine need to help him". "The paw patrol pups are getting ready to rescue Lego man / teddy etc" "What instructions will Ryder give them?" Simple games like "You have the red Duplo and I'll have the green Duplo, Who can build a tower the fastest? Ready, steady go!". You get the idea and I bet you do some of this already. It all builds opportunities for language comprehension and expression.