He's going to grow up really big! Such an awesome looking Uro. If you can, go with as big a cage as possible. At least 3 feet long, by about 15 inches high and 15 inches wide, should work for the first year or two. Yes keep the temperatures hot just like the adults, with a cooler spot too. He will be able to warm up better than an adult because of his size relative to the temperature, but he will also cool down faster. Give him a hut to hide in of course. And, if at all possible... NATURAL SUNLIGHT!! His little bones are growing quickly, and need to be strong. Natural sunlight and calcium intake will insure he has strong bones, because he's gonna get BIG.
These are just my OPINIONS. I'm not claiming to be an absolute expert, but I sure hope some of this helps. I've been following this on-going drama of yours with this little guy, and I want him to live!
-Paul
1.0.0 Mali Uromastyx
1.1.0 Uromastyx Geyri
0.1.0 Bearded Dragon
0.0.2 African Fat Tailed Geckos
1.0.0 Royal Python