Thanks for using the forum. I am happy to address your questions, and my answer will be based on the information you provided here. Please make sure you recognize that this forum is for educational purposes only, and it does not substitute for a formal office visit with your doctor.
Without the ability to examine your son and obtain a history, I can not tell you what the best management plan is for him. However I will try to provide you with some useful information.
The bump on your sons head could be a hematoma, or small bruise. However, with multiple episodes of head trauma, particularly since you feel his face pulls or droops, I recommend he be evaluated by a physician. It would be best for you to share your concerns with his regular pediatrician, who may then choose to refer you to a pediatric neurologist. I would take an appointment soon, rather than wait for August.
The biggest concern with repeated head trauma is of course a bleed in the brain though this is very unlikely given that he sounds otherwise fine and this sounds like it has been going on for a while. With the face pulling, one thing that would need to be ruled out is a seizure or other abnormalities of muscle, such as dystonia. Some of these are brought on by stress such as being upset; alternatively, it may be that when his face pulls (for whatever reason), he then becomes upset?
Since his symptoms are episodic, I recommend that you video tape him when his face is pulled so that your physician can see it if he does not have the symptoms while he is at the clinic.
Thank you for using the forum I hope you find this information useful good luck.