Sometimes the behaviors which seem odd to the uninitiated are coping techniques which comfort the person with ASD. People on the spectrum are often under an enormous amount of stress dealing with sights, sounds, smells, and the simple presence of lots of new people. It sounds like you've got a big family, and this young man is in a new country, speaking a language which is not his first tongue. These would be somewhat stressful situations for many, perhaps most, 17 year olds, but he's got an unimaginably greater amount of stress to handle.
I know you want the best for this young man, and I'm speaking gently....
What you see as getting too comfortable and behaving badly sounds very different to me. In our family, undesirable behaviors appear as a reaction to stress which has become overwhelming. Perhaps for the first few weeks he was able to keep things under control, but now it's all just too much. Generally, as hard as these behaviors are for bystanders, it's harder for the person with ASD.
Again in our family, and I can't speak for others, the least helpful action is to draw attention to these behaviors in the moment. He is already too stressed to control them. Embarrassment can be acute and further overwhelm him.
His parents can probably provide the best guidance. What did they suggest?
I agree to focus on getting your son some space. His frustration may be compounding your guest's difficulties. Could you find things for him to do away from the guest? Some reading material from the library about the challenges faced by people on the spectrum might also be helpful. Probably the behaviors you're seeing are not just based on lack of social understanding, but spring from acute discomfort.