I think you have every right to be concerned. But, assuming he's going to a half way decent school, I think you're being overly pessimistic about what is going to happen.
For a start, I would be very surprised if reception children are just expected to start school on xx September without any of the usual settling in/planning/ accommodation that happens. However, it's impossible for schools to plan alternatives right now as they don't know what the government guidelines etc will be by then. For example, if bubbles are still necessary, that will change how they teach, teacher allocation, class planning etc. That also then changes what settling in may be required. So, frustrating though it is, you need to give them time to work some of this stuff out.
In terms of his specific needs, of course you should have the option to provide some of this information ahead of time. Different schools/areas do it differently. At our school, we fill in about 3000 forms in which we flag any learning/physical/social challenges, are asked to provide any details of outside agencies who have been engaged with the child and also the details of any nurseries the child already attends. These nurseries are then asked to provide some basic information regarding the child's progress/learning within their settings. This informs things.
Incidentally, DS had been seen by SALT but discharged at the time of his admittance to school but his details must have flagged something on the system and in fact his nursery received an additional call from the our local educational team ahead of him starting school to ask for extra information.
Whether or not he will get an EHCP or not, if he has the challenges you are referring to, the school will notice (plus, they'll have the pre information you've provided). Most likely, they'll work with you to agree on what support they can and should provide. Whether these are offered from reception is likely to vary from child to child but certainly at our school, DS has been getting extra support from Year 1 informally and this was formalised in year 2.
This is all a lot of detail but my point is that while I don't think you should be complacent, unless the school has written to you to say "we're starting full time school on 4 September and this is his class", I wouldn't be assuming he won't be getting a version of the same support all children starting reception get.