80-85 degrees is fine for a 3 month old. I'd go closer to the 85 range and then around 90-95 when he gets older. At that age I would feed him 1/2 inch crickets. I think Veiled's are slightly bigger than Panther's in general, but the rule of thumb is for the feeder to be no bigger than the width between your chameleon's eyes. They do open their mouth when they get too hot and that's fine as long as you're not hearing any weezing/popping noise. Can you post a picture of your set up and your chameleon when you get him? It would give us an idea and help us help you if there are any issues.