If there was a way to know that then it would be the equivalent of turning whatever into gold. I know how you feel though, I could have written your post 2 1/2 years ago. I scoured the internet looking for surveys, stats and answers but in the end only time could answer the question. Your other posts mention positive features - receptive understanding etc. Ask your SALT if you can go on a Hanen course - has anyone mentioned this yet?
More Than Words is very good but can be a bit overwhelming in the early days as it tries to cover a lot of bases (recognising that different children need different approaches). But I think you really need to go on the parent training course to get the best out of it. Do you have confidence in the SALT you have at the moment? Building a relationship of trust with someone willing to help is one of the best ways to start thinking more positively. DS1 is verbal now at basic functional level and improving slowly over time . We still don't know what the future holds but then you never do even without ASD. His imitation skills are not great other than immediate echolalia - but no one I have asked has ever really known why this is the case.
I've seen far higher figures than that - from memory I recalled a statistic of 90% of kids with ASD being verbal, and on a quick google I've found a figure of 86% (with the criteria of the more than 5 words). Facing the complete unknown is very daunting. I agree with cyber that what you have posted about your boy contains some very positive stuff - the joint attention was what sprung to my mind.