Lovely to her about your ds, Sign.
We had the same dilemna with bsl and makaton, at one point ds was diagnosed with a hearing impairment and it seemed as though bsl was going to be the way to go, but he then had a normal hearing test, and we stuck with makaton!
I am now half way through makaton training, partly for ds, and partly because I am working as a 1-1 with a disabled child at preschool. I find that most of the children at preschool pick up makaton really quickly, and this was lovely when ds was there, as he saw other children signing as well.
Now at his mainstream primary (he is in a special needs unit, within the school) they sign during assemblies, carol services etc - it is lovely.
Ds has a couple of words now (he is 5) but they are very difficult to understand. He has a tablet communication device, which is amazing, and which we are all just learning to use, which I think will help him for the time he spends in mainstream, but the signing is a very quick and easy way of communicating.
Falling, I think you need to talk to the SALT about signing (like sign, i am surprised it has not been mentioned before), but also need a clear plan of how SALT, school and you are going to work together to support your ds with his communication. I have found that schools sometimes use visuals without much focus, and when it doesn't really suit a child, because it is fairly easy to do, and looks good (they have their place, but imo are often used inappropriately)
Your SALT should know of any local makaton courses.
www.facebook.com/pages/More-Than-Words-Charity/445034525630807 A facebook page for a charity which raises awareness and organises courses for makaton - they often have good makaton links and videos.
www.makaton.org/shop/shopping/browseStore/Free-resources The official makaton website - their resources are pretty expensive, but they do some free sign sheets, and if you like them on FB they send current signs to your news feed.
www.bbc.co.uk/cbeebies/shows/something-special something special - the website has a lot of info and the comic has loads of signs each month
At the beginning it is a good idea to think of, say, 10 words that are used often - eg more, biscuit, juice, finished, school etc and use them, whilst saying the word, everytime.
This video has some good signs, to start with, and it is easier to watch a sign being demonstrated than looking at an illustration!