My son has cerebal palsy and speech delay he smashing his head in frustation or when can't get his own way would this make his CP worse?

Unlikely. Head banging is not limited to CP kids. In a way it is just an amplification of the early infant fussing. I've never heard or seen a head banger with a real head injury. This likely caught on when he noticed it grabbed your attention. If you ignore it (don't lock eyes), watching him out of peripheral vision, for safety, it should fade over time. Teaching some sign language may help with the frustration.
Doubtful. Dont feed into the 'fits' but do work with speech or occupational therapists on other ways for him to express himself. May need a helmet if looks like he is hurting himself, which is possible but unlikely to worsen cp.
Inability to express. My bet would be that he can get overwhelmed when he can't express to you what he wants. I would work with speech therapy to see if there are augmentative devices (there are apps for ipads and other tablets now that can help) that can be useful to him. I would make a book of pictures of things he would most likely be thinking that he can point to at such times if you can't get another device.