Maybe start to use a hand sign (like ASL) for “need a break” - if adults use the sign and are responsive to whatever version your kid might use then he may learn to self-advocate in more ways as well. Hand signs can be extremely helpful in supporting the advancement of communication, even verbal. You can google “using hand signs in speech therapy ASD” for some good guidance.

If you can,create a quiet space with pillows and bean bag chairs he can have a few minutes to calm himself in. This has been pretty helpful for my son at school.