Toddler room is FREEZING

My son is 15 months old and his room is absolutely freezing. We’re living with my in laws right now temporarily so his room is actually 2 floors above us across the hall from his grandparents. I already absolutely hate being that far from him even with a monitor on but he usually ends up in bed with us because he still wakes up a lot at night (he’s been sick, teething, and now cold). Anyway, his room is fine at bedtime but by the middle of the night it’s a freezer. I’m hugely against space heaters in a young child’s room (and it costs a lot to run a heater all night) but my boyfriend is insisting it’s safe and it’s turning into a huge fight. We already made sure the windows are good and the heat from the furnace is going into his room it just doesn’t heat it. He’s got his blanket and wears Fleece footie pajamas but still gets cold. Im so close to just letting him sleep down there until winter is over but his dad would never be for that. What would you do? Are there safe heaters?