Could you put a crib mattress or air mattress on the floor in your room and tell him that if he wakes up in the night, he can quietly come in and sleep on the air mattress on the floor in your room? Or just let him sleep on the air mattress on the floor in your room for now and then have him transition to his room, with the option of coming into your room if he wakes up, in a few months? It may help all of you to get some sleep while at least getting him out of your bed.
Its okay if he crys it wont hurt him but you have to be consistant, and his Granma should do the same ,he is getting mixed messages, so unless you are consistant it wont happen.he will be like this for a long time.
Part of the problem is that he shares a room with his siter and when he cries and has a fit she wakes up and then it is a big mess. We have tried telling him if he can sleep in his own room for 5days we will take him to do something special, that he can do with his mom or dad just him and or or him and dad. He slept in his own bed on saturday with His Grandma and got up at 5:30 am and crawled into bed with grandma she is rewarding him with a new video. Last night he came home and i said okay time for bed he said my mommy I don't want to go in my bed I want to go in yours i said no more you need to sleep in your bed and then he started crying and screaming.
He is with you now though so you can take control of this, you talk to him and tell him he is going to sleep in his own Room like the Big Boys do,then say no more , make his room as inviting as you can with his toys and books and good covers on his bed. Take him to bed, read him some stories and tuck him in and leave the Room, you can leave the door open, if he yells and he will, ignore it,if he comes out take him back ,over and over if you have to.Do not get upset ,he is fine if you persevere he will get used to it, consistancy is the Key word here.If he goes to stay with your parents they must do the same or they will be sending mixed messages and you will be back to square one.