Why does your pale agama bask with his mouth open?

It's a good sign if his mouth is open under the basking light. This means he knows enough to open his mouth and cool himself off since he's a tad too warm. It's not a problem you should be worried about unless you see that he has a severe amount of mucous around his nose and inside his mouth. That suggests a respiratory problem which should be treated immediately- most likely with antibiotics.