It's great that your nursery is being so pro-active. I would still ensure that you're on top of the deadlines for the EHCP process though. Hopefully the LA will stick to the legal timeframes but many don't. I would also prepare yourself for a fight. Again, hopefully it won't be needed but many LAs turn down assessment requests, however compelling the evidence is.
There is some good basic info here. It also covers choosing a school with an EHCP:
https://www.ipsea.org.uk/Pages/Category/get-support
It's absolutely vital that when/if the draft EHCP is issued, you play close attention to Section F. This is the legally binding section of the EHCP - all support must be quantified and specified in here. Whatever school your DS ends up attending, this is where his support will be listed (eg how much speech and lang. he will get, what his classroom support will look like etc). If this section is well written then it may be that he could thrive in a mainstream setting.
I would also say that whatever decision you make, it doesn't have to be for the whole of his primary years. It is not uncommon for DC to move settings - either from specialist to mainstream or vice versa. The EHCP is reviewed every year - you can also request an early annual review if things aren't working out.
Have you looked at any schools yet? Either specialist or mainstream? If not, I would be making appointments in September. I would also try and book appointments directly with their Sencos if possible (although they can be very busy).
I take it you also know that with an EHCP you can request an out of borough school? So do look at any you feel has a reasonable travelling distance (your DS may also be entitled to transport).
You don't say if your DS has a diagnosis of a particular condition (although support is based on need not diagnosis anyway). If it is autism, my DS is in an autism resourced unit attached to a mainstream secondary and that is working well for him. It's worth looking at those too as they are offered at primary.
Your DS sounds like he is making excellent progress.