Technically Sid's first on-screen role was as a young King Tut in a documentary. I believe he was 11 at the time (?). He did have a teeny cameo role in Sammy and Rosie Get Laid, but Big Battalions and A Dangerous Man are his first "big" roles.
I don't think you can call it a "cameo" if you're an unknown. If Sid did a tiny bit in a movie NOW, it would be a cameo. In
Sammy and Rosie Get Laid Sid is a party guest. Sid was not an actor at the time but was acquainted with the casting director who was looking for an ethnic mix of young people. It's surprising he actually got a screen credit for that appearance.
Carol