Hey, good for him. He's trying to figure out spoken language, evidently, or at least playing with it. That's the same general way babies do it; they start out with babbling, and trying things out, and eventually start piecing things together. I don't know if you have one already or not, but if he wants to learn to use speech for communication, maybe you could get him a speech therapist? And there's lots of other options, too--sign, writing, symbols, pictures--if the speech doesn't work out. Speech therapists are trained in those, too. Just find yourself one who'll treat him with respect and keep you in the loop (you're probably his primary communication partner anyway) so you can work on it together. People often have this funny idea about autistic people--that once we're past early childhood, we don't learn much more than we've got--but we all know that's utterly stupid; of course adults can learn. Sometimes we're not even ready until we're pretty old... I wasn't ready to learn to drive until last year, and got my license at 25. Any younger and I would've been a road hazard. I think we're just on a really odd developmental schedule, with milestones stirred up and scattered around like a tornado in a baby book.
In autistic people there's often a stage of "functional echolalia"--using memorized phrases to communicate, like quoting from a book that's been read to you to communicate something that's similar to what the book was saying. You can get actually very creative with just memorized phrases; and they're easier than generating fresh sentences each time; so it's not surprising that a lot of people use that method to get ideas across. I wouldn't be surprised if some adults use this method still, and are considered fully verbal simply because they have such a large stock of memorized phrases and the ability to mix them up and use them effectively.