If your boy is in a crib I would give you the advice my Dr. gave me with my 1st. Kids are smart, they learn to do what it takes to get the attention they want, even if it means not sleeping. As parents, or grandparents we feel badly if our children cry for an extended period of time and we end up hurting them when we could help them. For example if you try to let you child cry them self to sleep and you wait for 2 hours and then finally go in and comfort them, than the next time they know that if they cry for 2 hrs someone will save them. I know it's tough and you might not watch him much, but if you're willing to let him cry through the times he wakes up, after a few times he'll realize that he sleeps at grandma's house. My son was 4 mo when we let him cry through the night and it took 2-3 nights of crying and then he slept through the night from then on.
If he's not in a crib let me know, I have another suggestion (one we used with our 21mo old).
A little about me:
I have 5 kids, my oldest is now 6 and my youngest is 3 mo