Do you not remember how growing up works? You don't do it all at once at some certain age. It happens little by little through the teens (and sometimes it's not done by the early twenties). One day you feel like an adult, the next day you're back with your dolls/hot wheels. Gradually you spend more time as an adult, less as a kid. But it happens in stages. He's eleven (you're not sure?). Cut him some slack.
He's just a kid right now not 13. I played some little kids games when I was 11 I have a mentally challenged and mentally ill cousin who also has down syndrome who's 15 and still watches toy story and a lot of little kids cartoons.
11 is still young. It's good that he's playing little kid games cus he still qualifies as a little kid! You should be glad he's doing that and not some inappropriate things, that 11 year olds do but shouldn't.