You can use a night time bulb to keep the ambient temperatures up during the night. Usually a 40-60 watt is sufficient depending on where you live. If you find it’s too hot then you could invest in a dimmer to attach to your lamp, and higher wattage if you need to increase. Never allow temperatures to dip below 75.
The reason why he might be staying under his cold side hide is because it might feel more secure than the one on the warm side. What are you using as your hides for each? You also want to have sufficient amount of foliage and cover(unless kept in a rack, but I’m assuming you’re not). It’ll make the enclosure feel smaller and safer. For your hides, try to make the hides identical, or make it appear that the warm side is safer and more hidden by making the entrance smaller or adding plants and leaves around it.
Remember that it’s only been 48 hours and your snake will need at least 1 week to adjust(some will need less, or more)
Also make sure that the enclosure is the proper size. A good rule of thumb is that your snakes length should be between one length of the enclosure(longest side), to the sum of the length and width.