I hate to say this because you're looking for help online but this is one of those situations where it's time to go see the nearest doctor. I can't even begin to imagine what you might be able to offer him at home that would be better than your local hospital.
Vomiting blood can be coming from the stomach/esophagus (ulcer or cancer), lungs (pneumonia or cancer), upper airway/back of throat (bleeding), nose (bleed), etc.
Severe headache has almost too many things to list (if you can post here, you can search online) as the possibilites are endless. But given what you've told me so far, I have to think of bad, deadly emergencies like cancer & meningitis.
Pardon me for asking (as I am not an infectious disease specialist) but how common is rabies? Mouth filling up w/water makes me think of someone who can't swallow saliva. Coupled w/headaches makes me think of infection.
Of course, a cancer in the throat could block swallowing and if eroding, could also lead to vomiting blood.
I'm sorry to give you all these bad thoughts but really, I can't think of much that anyone can do at home. It's really time to see a doctor, and if that doctor can't do anything, then get him to someone who can help him. But unless I'm missing something, it's beyond what you can find in a medicine cabinet. Someone who knows more/differently, please step in. God bless you both.
NOTICE: The information provided on this site is not a substitute for professional medical advice,
diagnosis, or treatment. Never delay or disregard seeking professional medical advice from your
physician or other qualified health provider because of something you have read on Wellsphere.
If you have a medical emergency, call your doctor or 911 immediately.