Welcome to the forum and happy holidays to you too .
You are doing a great job! Very kind of you to bring him in.
Your temps and humidity are fine.
Being you have had aquariums, same principles apply...dechlorinate his water and water changes, cycling of the water side of the tank, etc.
He will likely be very shy and afraid for a week or two. It takes them a bit to adjust to their new surroundings and to having people watch them.
The covering is a perfect idea. We recommend covering the back and 2 sides of the tank to help him feel safe. You can cover the front too if you'd like. Try adding an amphibian or reptile cave and some plants to hide in. You can even tip a planting pot sideways. be sure whatever you use has smooth edges...they are powerful jumpers. They love to hide in plants. they can be real or fake. You can use aquarium plants and/or terrestrial plants. I'm sure he'd love both . Be sure to rinse all fertilizers off/out. My Bully loves to duck and hide in his water plants. He will, however, probably beat up the plants a bit. It makes them happy though.
Be sure your land area doesn't have rocks small enough to accidentally ingest while diving at his food. They are aggressive eaters and will watch their food contently and then dart at it quickly. I feed mine from the top of his water cave.
What type of flooring or substrate have you used for your land area? Good recommendations are coco fiber (not husk) or plain plantation soil.
Foods can include: crickets, earthworms, night crawlers, roaches. Avoid mealworms bc the chitin in their segments causes constipation which often leads to intestinal impaction. Be sure the worms do not have any dyes or scents added to them. Cut them to the proper size. Their foods should be no larger than the distance between their eyes for crickets and roaches. Worms can be slightly larger.
How big is he? How long from nose (snout) to rectum (vent)?
If he is young/small he should be offered food daily. What he does not eat by morning should be removed from the tank until the next feeding time. Crickets can bite and the roaches will burrow and 'disappear'.
You can also try the feeding bowl method. To do this, use a slippery glass or ceramic bowl and dig a hole in the substrate to push it down into so it's at ground level or so there is just a lip above. Put the feeders into the bowl. He will learn where they are.
Supplements...all amphibians need nutritional supplements or the will become sick or will develop Metabolic Bone Disease. You will need an amphibian or reptile calcium with vitamin D3 powder and an amphibian or reptile multivitamin powder. Lightly dust the feeders (insects/worms) every other day with calcium/vita D3 and one day a week with the multivitamin (MVI), but not both of them on the same day.
You can use the shake-and-bake dusting method...use a baggie and place a pinch of the supplement into it. Place the feeders in the baggie and gently shake. Only a fine coating is needed and recommended. A coffee cup also works well.
Ah yes, and be sure to have a weighted lid or get some aquarium lid clamps. They are stronger than they look and can leap 3 to 6 feet .
Hope this helps .
P.S. I love my bullfrog. Once he starts to relax and settle in he'll be fun to watch. They can be quite energetic and their swimming abilities are awesome. Mine dives off of his waterfall and does 360 degree turns in the water. He's an awesome hunter, and he loves to play peek-a-boo between his water plants. I've had mine a couple months now, and he comes to the front of the glass to watch us now .