I am the king of slang. I didn't realize it until I joined the Peace Corps back in '91. As I engaged with Poles throughout my time in Poland, I found that I was using much more slang than I realized. In some ways, speaking to my son is the same experience as it requires the same level of "foreign language awareness".
A key to remember when talking to people on the autism spectrum is to minimize your use of slang. Keep it simple. Keep it concrete. Keep it real.
Over the last 2 years, M has become more aware of 2nd meanings and slang. There still is a strong propensity toward literal interpretations but he has become somewhat more flexible. That has led to much more entertaining conversations. Last night was a good example.
He was putting on his pajamas and getting ready for bed. I walked in and found him standing in his bedroom in his boxers. He was pulling on the front of his boxers incrementallly with both hands making funny "pfff" noises. My wife and I asked what he was doing and he looked up at us with a twinkle in his eye "I'm boxing".
That skill will serve him well in the future.