I would suggest ringing round the schools and asking specifically if they have had children on the autistic spectrum before and what level of support they would be able to put in place for a child who does not yet have a statement. Here schools have to put in up to 20 hours 1:1.
A diagnosis will bring some help from an autism outreach service (if thats the diagnosis!)
You could apply yourself for a statutory assessment of his special educational needs - there will be info on your council website but also look at ACE, IPSEA and NAS. Failing to make progress is a ground for a statutory assessment so you could apply for a statement.
However it would depend what help that would bring - unless you have an array of specialist provision locally the assumption will be he will stay in mainstream and a statement may bring more hours of 1:1 with a TA. But then the TA may not have much experience so your child does not learn. So IMO getting a supportive school who is experienced or very willing to learn quickly is key.
It is quite possible that if he is on the spectrum the difficulties that throws up for learning has caused him to fall behind rather than him having GDD. Children with autism tend to have delayed language skills so of course that makes everything happen later and also don't learn in the same ways eg they imitate and observe far less. They also tend to be absorbed in their own interests, more distractible and learn less through play etc. So as a result they fall behind.
ASD children can often have spiky profiles eg they might be ahead in some skills and very behind in others.
Your council will have a parent partnership officer who can send you a list of schools and may nudge you in the right direction. Or ring your local NAS branch and ask for parents recommendations of schools.
Usually you can tell by the reaction of the receptionist when you say you have a child with suspected ASD and want to come and look round!
Often money for SEN is distributed on deprivation indexes so the schools in the less well heeled places with the more mixed intake tend to be the ones with the most experience of different learning needs and the most money to take on 1:1 staff etc. Often village schools really struggle to pay for the extra staff and training etc.
Late walking and food issues are common with ASD.
You can also self refer to the local authority educational psychologist - its a good idea to get on that waiting list asap as they can advise on what will be needed and settings.