Answers:
-just a baby, my guess anywhere from just recently out the water to no more then a month or two.
-he looks very healthy and the fact that he ate no problem even being in a tank that is too big, shows that. And no, if he is eating, no need for a smaller tank.
now for pointers.
-cover 3 sides of your tank with Smth not see through, it will help him feel more safe and you will like the look of the tank better

cling fish tank backgrounds work excellent for that.
- raise humidity to 70-80, it would help if you completely cover the screen top and cut the hole for the lamp + a bit around i, perhaps also look into getting a fogger ( or making one).
- temps, you gotta keep it in a proper range, keeping in low temps long term will cause lots of health problems that you don't want to deal with. Keep 76-78f night, 80-82 day.
the chances are you gauge is not correct, analog ones are very well known for showing whatever they feel like lol get digital, not nesseserily reptile specific, in fact regular home digital ones are cheap and more accurate, I believe walmarts sell temp/hydrometer combo for 7-9$. I suggest before adjusting temps/humidity you figure out if your reading are good.
- 100w infrared lamp seems very excessive, especially having a heat mat ( well it probably don't do anything anyway) and 10g tank. So once again I suggest you check the readings.
- the best way of dealing with temps and humidity issues is to get hydroterm by zoomed(50$amazon), you plug your heating things in as well as fogger and live worry free ever since
- you need to get ca/vitd3. powder and separate multivitamin powder, dust crickets 2xweek, multivitamins once a week, but not on a same day. Whatever pet store says in regards that sell a,ready dusted crixs, etc that's bs, you don't know what exactly did they use and by the time you feed your guy all or almost all powder is gone anyway.
- while the guy is little and obviuosly eating, try to tong feed, you will avoid lots of problems down the road. The best staple is nighcrawlers, for him you need to cut it to small pieces ( about the distance between his eyes). These worms you can get anywhere where they sell fishing supplies, just make sure they are not dyed.
- water.... The best is to use tap or spring water and use these drops that you have, water conditioner. Purified is lacking minerals that the guy need.
The biggest pointer is take lots of pictures! They grow very fast, in 4-5 months you will have almost adult sized frog

enjoy, very very pretty frog!