I think it will very much depend on the mainstream school as they do vary massively on how they treat/ support kids with Sen despite what may be in an ehcp. As a first thing I’d visit the potential schools and be completely honest with them about your sons needs, if they try and put you off then you can take that to mean that they are never going to do the best for your child and it’s probably not the best place for them, your child deserves to be somewhere where they are wanted. You will also find you get a gut feel/vibe from visiting a school, speaking to the head/ senco etc.
Regarding behavioural problems, your son might not have them now but these can develop after years of being in an unsuitable placement and not being supported properly. That’s what happened with my son.
Once you are in mainstream it is really hard to get out so it’s not just a chance of trying, not quite likely it and then switching, you will have a massive fight on your hand,need to have loads of evidence and probably go to tribunal. Even then it may not be guaranteed unless you have lots of evidence. Another person mentioned above it’s harder to switch from specialist to mainstream as the child may be more behind, it may be true that the child would be more behind but the actual switch would be easier in that you wouldn’t have to fight the la for the move so it’s likely to be agreed a lot quicker without tribunal.
My son started in mainstream without any obvious needs, he was meeting all his milestones with speech, reading, toilet trained etc etc. So we didn’t have the choice initially but things went wrong from year one and he was diagnosed with asd age 6. We had 3 and a half years of absolute hell and he eventually got an ehcp but things had gone to far to still didn’t work and then he eventually moved to specialist but it was a massive fight even though we had lots of evidence that the school couldn’t meet needs.
The specialist school he’s at now is for kids with asd that are at mainstream levels academically however unfortunately this type of school doesn’t exist thst I know of for reception and is made up of kids who have tried and failed mainstream in some way.
I think mainstream is definitely better academically in general if your child can manage it, but if they end up not managing then the academic side being better becomes irrelevant. I also found as my child got older the difference between him and others opened up, so not so noticeable in nursery/reception but by year 6 very different and doesn’t really fit in.
As I said above my best piece of advice is to visit and speak to the school and trust your gut feel, it will almost certainly be right. If available in your area then I’d look at units attached to mainstreams as with those you would get some chance of trying mainstream, but again you’d need to speak to the school as the units can vary.
Above all don’t fall into the trap of thinking that the la/ school want the best for you child and trusting what they say. There are some good people about but I was very naive when I started out on my journey and I found out over a period of about a year or more that not everyone wants what’s best for your child or will do right by them, so you need to be there to advocate for them.
Lots of children do go to mainstream and it works out well, so I don’t want to put you off, but pls go into with your eyes wide open about what might happen and trust your gut feel