4 year old having nightmares, but only in HIS room?

He has them almost every night, for the past few months. He wakes up screaming and crying.

His grandma hasn't mentioned him having nightmares when he spends the weekend at her house, we've lived in this apartment for 1.5 years now, and he doesn't have them when sleeping in our bed. Haven't noticed any patterns with food, bedtime, etc.

I've tried asking him what the bad dreams were about, but he can't/won't tell me. Anyone have any advice?