Hi, we went through the same exact thing. We had the same situation - grandparents watched him during the day and rocked him to sleep for naps. And until he was a little over 1 I would rock him to sleep at night and then put him in his crib after he was asleep. But it got to the point that it wasn't manageable. And I had heard from friends with older kids that you need to get it under control around age 1 or you could have a 4 or 5 year old who has bad sleeping habits, which we wanted to avoid.
So, we established a normal bedtime routine. Before bed we sit in a rocker in his room and sing songs. Then when he's drowsy but not asleep he is put into his crib. The first few nights were awful because he would jump back fully awake once he got into the crib. I would pat and rock him but not pick him up out of the crib. When he was asleep or near asleep I would leave the room, usually prompting crying. It was awful.
We'd take turns going in to reassure him, but didn't get him out of the crib.
It took a few weeks but each night the crying would lessen until no more crying. Mind you, we would take some steps forward and have GREAT nights, then a few weeks later (after I thought all was going to be smooth forever) he would regress and cry again. But, it worked out.
Even the nights he would cry before falling asleep, he still woke up my happy little munchkin. So, it didn't seem to damage him. And we would go in every 5 minutes to check on him (sometimes less).
Now he's a little over 2 and does great at bedtime. Good luck.
Also, try to wear him out the hours before bedtime so he is really tired. Then he will want to sleep when he goes to bed and not have the energy to cry. : )