You are not the only one at all with these issues, this is extremely common, especially after the dragon has been given medications, they're hard on their tummies. He'll need at least a 40 gallon breeder tank soon, around 5-6 months, but that isn't the problem...honestly, I know you said he "doesn't like heat", but why do you say that, or what makes you think that? And I forget, what type of thermometer are you using? My honest guess is the same as AHBD already mentioned, I think the temps are not hot enough. Most babies and juveniles are not comfortable on their main basking spot until the temp is between 105-110 degrees (110 is the max temp it should ever be anywhere inside his tank), and even adults need their basking spot between 100-105. The Hot Side Ambient (air) Temp should be between 88-93 degrees, and the Cold Side Ambient (air) Temp should be between 75-80 degrees...
If you're using any type of stick-on thermometer and not a digital thermometer with a probe or a Temperature Gun, then that could very well be the problem, as they can be off by up to 20 degrees, and you cannot measure their Basking Spot Surface Temperature with one, only Ambient (air) temperatures....
If he is on his Basking Spot and the temperature is between 105-110 degrees, and you see him opening his mouth a lot or darkening himself, these do not mean he doesn't like it or that it's too hot, these are common misconceptions of new owners and sometimes they think their dragons are too hot and they drop the Basking Spot Surface temperature because of it. They open their mouths as a way to regulate their body heat, it's not because they are too hot, it's perfectly normal and they should be doing it often while basking. And if they turn very dark while on the Basking Spot and then turn light and bright when you take them out of the enclosure, this is because while they are Basking under their lights they are darkening themselves so that they actually absorb more heat and
UVB light, just like a black tee shirt absorb heat in the sun. So when you remove them from their tanks they immediately brighten up because they are no longer under any lights and don't need to attract anymore light/heat...I just wanted to make sure that wasn't what you were thinking, we hear that all the time.