When chams open their mouths usually thats a sign it's too hot for them, so they open their mouths to regulate their body temperatures. It's completely normal so don't worry just be sure his basking hotspots are not over 90 degrees. Also have multiple levels so your cham can choose the right temp.
You need to double check your temps.
Be warned, those cheap thermometers u get from the pet store are less
then worthless.
Use a accurate digital thermometer with a probe and place the probe in
the basking spot.
Take several reading over the day, or simply attach it and run the wire
out of the cage.
I have all my thermometers on top of the cages at all times, and the temps do fluctuate over the day quite a lot.
When your cham is sitting under the basking light with his mouth hanging open, this means he is too hot.
They do not always have the good sense to simply move