I'd like to know my friends peculiar disorder?

So basically, I'm in 11th grade at the moment and I had a friend who came to my school in 9th grade and was in my class for the two years he was here, from ninth to tenth then he left.

He was a very strange kind of person, in a way where he would act a lot younger than he is, like he'd act like a 7 year old, and it wasn't purposely. At first I thought he had autism, but he didn't seem to fit in that category because he was very intriguing, a tad bit shy but very sociable in an odd way. Kind of the way a 7 year old is. He'd say things like "Im a flying helicopter!". Haha. Thing is he was picked on a lot because of his immature behavior. But im 100% certain he wasn't faking it.

He had certain physical differences, like a bigger head and a weird way of walking. His voice was a tad bit high. Me personally I loved him because even though I couldn't speak to him and have a real conversation he was a decent bloke who needed some help so I let him stick by me and my mates.

But it was odd. Really odd, I'm sure he had a certain disorder and I was hoping maybe one of you people here would know what it is...? Like I said, he acted way bellow his age, had a slightly bigger head than most guys, walked oddly and spoke with a high pitched voice, not too high but a voice like a 7 year old would have. And I doubt its autism because he never threw tantrums, was as sociable as a 6-7 year old boy and never really showed symptoms of autism. And no he didn't have downs syndrome.

I still miss the guy and was happy to have had him 2 years in a row in my class. Now can anyone figure out what this disorder is?

PS: He had a surgery done in his legs right below his calf, I don't know if that would affect anything.