Predications don't mean very much. This is just the average grade achieved by children with the same SATs results, that is all. Its a national formula. Most people don't get the average do they? That is just the mean, most people get something else, better or worse. It's just a rough idea.
If your child gets grades this low, I am afraid the news is not good. There are jobs and apprenticeships at this level, but the competition is intense. In fact, there is more competition at this level than at top levels.
The best thing you can do for your child is work on giving him the edge. What other skills can he develop to put on his CV? Sport? Team games? Music? Art? Cooking? Gardening? These last two could be quite important as a lot of students with low GCSE grades end up in these sectors, and experience and confidence will help.
What is his communication like? Upright posture and clear voice help too, and the right amount of eye contact ( obviously, the "right amount" is going to be different for an individual with ASD)
The biggest challenge with students at this level is, in a system focussed entirely on GCSE results, to raise them to understand that there are far more important things! Kindness, trustworthiness, common sense - these are all far more valued in adult relationships than exam certificates. Please make sure your son knows this