He is shaking really bad, foaming at the mouth, and vomiting. Please tell me what it could be and how I can help him

You MUST take them to the vet as a matter of urgency!!

My best guess is rabis... Get him to the vet before something bad happens to you or your family.

Foaming at the mouth and vomiting could also indicate that he ate something he shouldn't have.. I know toads have that affect on dogs if they manage to eat it.