My son has verbal dyspraxia as well. (well we think he does- no-one has really assessed it properly). Well done for getting in there early though (3 and half is very young to diagnose it). Have you seen the apraxia-kids website? It's great. I'll post a link later if you haven't (sorry rushing off to school in a minute).
Don't really know where to start. Are you having regular speech therapy? This is essential for verbal dyspraxia. We have now given up on the NHS and have recently employed a private SALT. You may have more luck than me as my son is also autistic so I suspect he isn't an efficient use of resources. However a little girl I met who "only" had verbal dyspraxia (very severe) also had been unable to access speech therpay via the NHS and had to go private. IMO (and that of non-NHS therapists and other parents) you really need contact with a SALT once a week for verbal dyspraxia (with some sort of practice daily).
Have you been given any exercises to do at home? Mr Tongue, licking sweets off things, blowing out candles, etc? I can give you a whole pile of those sorts of things later if you haven't been given anything yet. In the UK they mainly use the Nuffield system for verbal dyspraxia- again I can give you more info if you haven't seen it yet. (The Nuffied speech and hearing centre can be a good place to visit if you are in London- although its a miserable building).
Verbal dyspraxia is a long hard slog to correct, but most of the parents I have spoken to found that after about 5 or 6 their children were speaking reasonably clearly.
Does your son have other dyspraxia type problems? Eg with fine motor skills. Actually I'm beginning to wonder whether my younger non-autistic son has verbal dyspraxia as well as he seems to have remarkably similar speech to ds1. So I could be joining you on the slog with him as well
Sorry I will write more later- and can give you lots of links, but have to go and get ready for school. I disagree with one thing that your SALT said- if your son only has verbal dyspraxia I think it is absolutely certain that it will be sorted out to some extent. By which I mean I am sure he will be able to use speech as the main means of communication (and will probably end up sounding pretty normal if not totally normal). Don't worry about her "no certainty" bit- I don't think the professionals realise what sort of effect comments like that can have on parents. Later on I'll give you a link to a page that has a boy wth severe verbal dyspraxia talking at various ages- you can hear there what great improvement he makes. Brings tears to my eyes