Oh, that's tasteless. There is an unspoken list of taboo topics when it comes to jokes or pranks: death and dying, disabilities, pregnancy, abortion, sexual assault and abuse, terrorism, school shootings, and Hitler/Holocaust/Anne Frank.
Your unfortunate prank had real repercussions with your boyfriend's parents kicking him out. Right now they aren't supportive of your relationship, although he still continues to see you. The best you can hope for right now is polite acknowledgment, and you get that by consistently behaving in a responsible and mature manner. You may try to write them a sincere apology letter with a card, asking for personal forgiveness. Tell them it was an immature joke in poor taste and you didn't know what you were thinking. You didn't understand the ramifications of your actions; if you could undo it or make amends you absolutely would. Say that you are sorry. Do not expect that it will automatically result in the desired consequences but make the effort. They may treat you poorly but don't return the ugly behavior. Remember who caused this and be the bigger person.