Is your lamp regulated? I would suggest getting digital thermometres for each end that you can just leave in place and check temperatures easily at a glance. They arnt very expensive. I wouldnt use just a uth soley because as you said, the ambient temps would be too cold. but it could be something to look into. Its fine for the ambients to drop within a certain range overnight. If 89 and 73 were your overnight temps and 93 78 your daytime temps, that doesnt sound too bad at the moment. Nothing that would make him sick imo