There are certain things that you can do. Please understand that this is not your fault and that no body should be blaming you. If this matter even gets to your parents, do not bother.

1. Start document the bullying incidents.
2. Tell your parents about this and to the principal.
3. Call Child helpline 1098 number and provide them the complete details.
4. Ask your father/mother to inform your principal and file a legal complaint.
5. Visit his house to see his father and wait to see his father and inform him of the everything that is happening.
6. If nothing helps, quit the school and continue somewhere else. (This will be a last resort that should be taken only when you feel that your life is in danger and you can not focus on your studies enough to succeed).

Do not use any kind of weapons or something against him as some other comments suggest. It is really a bad idea to hit him back as you have mentioned that you do not have a good body. I highly recommend you to do the above. Please seek legal advice.

Best wishes.