Congratulations, he's a beautiful froggy.

This is just my opinion, based on my own experience with bullfrog tads/froglets nearly 30 years ago, but IMO, there's no real reason to lower the water level, as long as he has a land area that he can get onto easily. Bullfrogs are highly aquatic, and should not be at risk for drowning, unlike more terrestrial or arboreal frog species. In nature (also personal observation) they usually hang out at the water's edge, and leap in and swim to the bottom as soon as they're startled. They also sometimes like to just hang out in the water, and he would likely be more comfortable doing that with more depth. If it were my frog, I would also probably add some water plants, just to give him more cover and places to hide.

The water depth in the earlier pictures looks just about right to me, and similar to what I had with my own baby bullfrogs way back when. (I apologize if I'm confusing you with conflicting information, just wanted to give you my $.02.)