My son is now 20 years old. We taught him to play when he was 3.5 years or so. The first game was how to crash car. He liked cars and trains but only swiped them in front of his eyes and had no idea what else to do with them. We set up crash sites all over the rooms and furniture. We played as though we just loved it and prompted him. It was fun--fun for us, fun for him. And then he started crashing his car, like every kid does. This expanded to cars on roads we built, tunnels we built, etc. All this took time (like ALL DAY LONG) but he figured out how to play with cars, trains, etc.
We also had other games going. Tweenies and Teletubbies were "in" then, so we played with those. He was fascinated by the tele tubby toaster but only skimmed with it, so we incorporated it into the play but tried to keep him from just swimming with it. Worked well....went onto other things.
As for cross-play, we started videoing us (as in me and his favourite therapist, who was a very young kid training in Verbal Behaviour) integrating one set of toys (e.g.. trains) into another set of toys (eg. Teletubbies). He liked watching the videos we made and imitated them. We then encouraged him to generalise to other settings, other toys. Whole thing took a lot of time...say, my whole life.
He has a very flexible mind now, not at all rigid. He was non-verbal at diagnosis at aged 3.2 months. He is now fully verbal and loves languages. He is still autistic. He was like the boy in Daniel Isn't Talking (my novel), except he didn't learn as fast as the fictional character.
These days I am helping him learn Python (a computing language) and accessing free learning from portals like Udacity, Khan, and MRUniversity.
I can only do this because I can work a little around his needs and because my husband has a full time job. My mother was a single parent--she would never have been able to do what I do for my son and it often worries me what other parents must be thinking, feeling, worrying about, as I worry.
I write about autism sometimes on my blog (www.martileimbach.com). I don't often post on forums. I was searching to see what other parents with adult children with autism are up to and I saw this post about play.
My own experience is that my son learned to love playing...and we know that we can help kids so much easier as the plasticity of the brain is just more extreme at young ages. I'd try to help teach play skills as soon as possible. You may meet with resistance, which means you then tailor the way in which play is taught through another channel. Yes, you have to be very inventive and tenacious and frankly extraordinary. I wish it were easier.
My son wasn't a superstar learner, nor was he always easy. He used to tantrum at the notion of getting out of the car (he wanted to stay in the car). Terrible, crazy fits of crying. It was rough. I know just how hard. I wish you the best of luck in helping your child. Nothing could be more important and you will find the rewards are amazing, and they last and last....Marti