ASD is a communication delay - in very specific areas of communication only rather than a more general communication delay. Language delay is a global delay in ALL areas of language acquisition - is this really your child?
It may be just very specific areas of language that you need a SALT to really drill into and help you sort out iyswim.
Also specific sensory issues are not part of ADD, (which again could be considered an overall delay) but they are part of ASD.
When younger a LOT of people (and more recently a new GP - gggrrrrrr!) mistook my son's sensory problems for ADHD when actually he very much isn't. Put him in the right environment and his concentration is vastly superior to most of the population's - it's just a shame that environment wasn't a busy urban one, or your average nursery/mainstream school.
For kids under about 7 I personally feel it can take a very skilled clinical team to tell the difference in SOME children between sensory processing disorder (often viewed as a subset of ASD), add/adhd. For this reason it may be worth waiting a few years and revisiting the diagnosis when he's older. My original diagnositic team wanted to take another look at my son when he got to 12! Great from a clinical perspective, but NOT helpful from an educational one. It did bring home to me that no diagnosis is necessarily written in stone, and that changing/removing them as a child develops is something that is far from a rare occurence in complex cases.
Educational provision is by law supposed to be based on NEED, not diagnosis, no matter how LA's try to wriggle out of it, so as a first step I'd apply for a statement, while the case law still applies (the system changes in September) so that if you have to go to Tribunal for that SALT unit place you want you can and stand a good chance of getting it.
The IPSEA website has excellent model letters to help you do this today.
Only after I'd done that would I go back to my GP and ask nicely if I could please have a second opinion - getting a second opinion is easy to do on the NHS - it's just with waiting list times, I'd go down the statement route for a belt and braces approach to getting that school placement in time.