First of all, wait till your boyfriend gets a job. He could take up a part-time job or freelance while completing his Phd. If you have no qualms about supporting him financially, go ahead with your marriage plan.
Ask a relative, who understands, to talk to your parents. If that doesn't work, ask the guy's parents to have a face-to-face conversation with your family. You yourself should try and explain the situation to them in person (though I would suggest you formulate a back-up plan to get out of the place, in case your family doesn't let you or gets physically abusive). You could also try convincing your mom or a brother first, whoever is closer, so that they would try convincing your father. If you are comfortable, pour your heart out in a letter and mail it to him.
If nothing works, the final decision rests on you. You could let your family control and manipulate you and decide against marrying him or you could tell them that it is your life and as an independent adult, you have every right to select a partner and that you aren't letting them dictate terms. Either way, it is going to be tough on you and everyone around. You need to keep in mind, as pencil said, that sometimes you can't have your cake and eat it too.
A lot of families tend to manipulate children and make them feel guilty to get what they want. Emotional blackmail is the norm and the aforementioned dialogues are used in 90% of such cases. You have the keys to your emotions. Don't give them to others and enable them to cause you stress. Stress isn't a solution. Besides, it isn't your fault that someone is upset. You only owe love, respect and gratitude to your parents, nothing more.