Couple of observations - The plant has really large leaves for a lemon tree. I'm thinking it's lived its whole life in a greenhouse up until you got it. Controlled temperature and humidity, lots of water and fertilizer. As it grows, it may put up smaller leaves that are a little lighter in color, and those leaves will tolerate more sun and heat. I'll admit I haven't seen a Ponderosa lemon for a long time so they may have bigger leaves, naturally.
Are you watering it thoroughly in the mornings? You need to water until the water flows out the bottom, and probably every day is not too much in hot weather. Did you break up the old root ball a bit when you re-potted? If the root ball dried out at any time, it's sometimes hard to re-hydrate the potting soil. The water you pour in will just run right by the old root ball and out the bottom of the pot. Could you stick something like a pencil well down into the soil, where the old root ball would be (not into the new soil you added when you re-potted). If the wood near the pencil point isn't moist when it comes out, your root ball is too dry, and you need to set the whole thing into a bucket of water to soak.
You could maybe get something thick and light colored to wrap that pot in? Or maybe spray paint the pot it's in now a lighter color? You can get good paints that stick to plastic.That pot looks plenty big for your plant. But citrus generally send out feeder roots pretty close to the soil surface so one way to keep it's roots cooler is to use a lighter color mulch.
Did you try acclimating it slowly to the sun? I'd put it somewhere it will just get the early morning sun for an hour or two. Leave it there for a couple of weeks, then move it a little bit further out from under the shade. Also it's cooler under the shade of a bigger tree, so if you have a large tree you can park it near, to do your acclimating, that's a good spot. Big trees transpire a lot of moisture, cooling their surrounding air just like a swamp cooler does.
