Partner Sex Help!

Usually my partner and I have sex in a car, as we both still live at home and it is what is most accessible to us. It's always great and he never has any problems. 
But when we do have the chance to do it in a bed, he doesn't stay hard for very long and isn't able to finish. 
It's really starting to frustrate him and making him feel like he is constantly disappointing me (Which I reassure him that he isn't, I'm just frustrated for him) 
Does anyone have any suggestions as to why? I thought it may be because the temperature in a car is pretty much the same the whole time, but in a bedroom there can be aircon or fans that's maybe throw him off. 
Any advice or tips would be great!!
Thanks 😊