Hi. I am a mother of a 2.5 year old. I have a similar problem. I let him sleep in my bed when I left his father, and I believe it was more for me than for him. We live at my mom's house and share a room. He has been in my bed ever since. I will be moving in a few months and my sister suggested getting his room all fixed up and decorated before he even sees it, and since he will have his own rrom then, he might be excited about sleeping on his own bed. It is funny though, that when he spends the night with my mother, he sleeps by himself, in a pack-n-play. She has helped me by getting him to sleep by himself, So that when I'm ready, it may be easier for me to get him into his own bed. Its a transition. I would suggest asking your parents to let him spend the night (if he doesn't already) and make sure that he sleeps by herself. I believe that because you are not there, it may be easier for your son to try to sleep alone. Grandmas are really good at getting things from grandchildren, as grandchildren are more likely to do things differently with grandparents. While you may not be moving, it might not hurt to try new things. And its NEVER wrong to ask your parents for help. After all, they did raise you!! :)
If he does begin to (or already) sleep by himself at grandma's house, then I would try putting him to sleep and then moving him to his own bed. And if he wakes up in the middle of the night, then let him fall back to sleep and put him back in his bed. Once he makes it sleeping in his own bed, congradulate him on how well he did sleeping like a big boy and maybe take him out for breakfast as a reward.