Hi fnm 
My DD is 4.11 and has a brain malformation (widespread subtle cortical dysplasia). It was only picked up at 2.9 that she had SN, and we didn't get her brain malformation dx until she was 3.5, so you are ahead of where we were at your stage - well done!
DD1 had some concerns in infancy, but they were dismissed by HCP, she was late in meeting milestones, etc., walked at 23 months. Tt wasn't until she went to preschool that they approached us and said that they felt she was 'behind' and could do with some 'extra attention'. She started falling over for no reason a month later, and she has since been dx with epilepsy.
DD1 got funding for 1:1 around 8 weeks after starting preschool, and she really did need it.
She knows her colours, numbers, etc., but is a wobbly walker (blue badge, just been uprated to High Rate Mobility) and falls, trips, stumbles.
She has low muscle tone, and she has delays in all areas. She is ataxic.
We could have pushed for MS education, and she would have needed full 1:1 support, OT visits, possibly some physio visits, SALT visits, etc. However, the LA offered us special school, and we accepted. It is wonderful!!
Two things:
Do not wait for statementing to happen around you. Write the letter, get it done. This puts you in the best place to choose your DS's education - if I had waited for my Inclusion co-ordinator to do the statement, all the special school places would have been taken.
Bear in mind that for DD1's year, she is the only child who started Year R who went to Mainstream nursery. All the other children had been at SN nursery. So if you send him to MS preschool/nursery, you need to be prepared that as nice and kind and well-meaning as they will be, they are likely to have limited experience of children with complex needs such as your DS and my DD. That is OK as long as you are prepared to bang your drum and make your DS known to the LA SEN officers so that they don't forget him.