Look at PECS Courses if there is one in your area. I wasted too much time waiting for professionals and went on one of these courses. If the cost is an issue they have a charity arm that can offer reduced cost places.
Also could try Makaton - didn't work for DS as he wasn't engaged with adults enough to watch them but does work for some children - you might be able to get Makaton resources from library or from Something Special on Cbeebies website / DVD. Sometimes councils or speech therapy run courses for parents
Also sparklebox is a website that does free symbols you can download and mrsriley.com you can make your own quite cheaply.
My son actually understood alot of words we found out once he started to talk - he just doesn't see the point of using language - the motivation is not there. This is common in ASD - PECS starts by teaching a reason to communicate eg you want this you have to ask for it -and that could be by using a sign, symbol, gesture or speech.
Ask for speech therapy to start now you don't need a diagnosis for speech therapy just a proven delay.
My son is very passive too - but if you stand back and watch what he's drawn to there will be reinforcing things - for my son it was anything with electronic buttons or a screen! Just put him in the room - see what he goes for and scoop it up into a box and watch what he goes for next. Then keep those to work on his communication even if he just gestures for the item thats the start of meaningful communication.
You might also want to look into ABA (expensive but lots of evidence behind it) or play based therapies eg here as you are down south.
You should be able to look at some ABA or PECS on youtube.
There are some internet ABA resources coming on stream now but I have no experience of these so cannot give recommendation one way or another eg here
Go and look at schools and nurseries and find out if any speech or language or ASD units /nurseries in your area. Specialist placements can start at 2.5 years but it takes most of us alot longer than that to fight to get a child into one. Especially if you have other young children getting into a specialist nursery where teachers can take some of the load off you is worthwhile - it doesn't mean your child will have to stay in special school many go just for a few years until their language improves.
ICAN and AFASIC are also websites you might want to look at.
Another way of kickstarting support and diagnosis is to start the process of getting a statement of special educational needs - see IPSEA website for details. If they agree to assess your child (and often they refuse and you have to appeal because delays save them money) then there is a statutory timescale they have to keep to and they have to get reports from different professionals eg paed, educational psych, speech therapy so it can actually make those referrals happen faster - and open up the chance of a specialist placement.