D.C.
He may need a change in his schedule. My son just turned 1 this week and his naptime has changed a bit in the last couple of weeks. He's just not tired as early as he used to be and trying to put him down when he's not ready is not going to work out. He wakes up between 7 and 7:30 and he takes a nap between 12:30 and 1pm. He'll sleep for about 3 hours. Then, he goes to bed at 8. If I watch him and follow his cues of when he's tired, he goes to bed with no problem.
I wonder what his napping place is like at Gramma's houses. If he still naps well at home, it could be because it's more comfortable there or darker, quieter, more condusive to sleep. Check out his rooms and cribs at Gramma's houses and try to make them as condusive to sleep as possible. Talk to your father and try to see if he's doing anything different with him than your mom and mil that helps him sleep better. Maybe he tires him out more or has a different routine to settle him down before a nap.
Good luck.