I will also agree to put the frog back where you found it.
It is use to being in the wild, it will be very stressful for it to try to adjust to captivity.
And too much stress can kill a frog.

I strongly recommend that you let your son do some research on what kind of frog
he will like to take care of.
and get one that is captive bred that is so much better for the environment.
Currently wild fog populations are declining and need every bit of help we can give them.
Have him save up to get the correct enclosure and support equipment needed to keep the frog
happy and healthy.
I tried to rescue a american bull frog and sadly it did not acclimate to captive living.
My self I am waiting until my vivaruim is well stablished before I even get my frogs.
I just finished getting the water part of the enclosure squared away and some feeder
guppies are calling it home for now.
I plan on getting a few whites dumpy tree frogs for now.
I have been keeping aquariums for the last 20 years and have kept African dwarf frogs
along with some non aggressive fish like neon tetras , ottos , cherry barbs , emerald corys and amno shrimp.

I hope your son will understand that the wild frog may be really cool but it really wants to be
free.
Until you release it handle it as little as possible and make sure you wash and rinse your hands
very well before handling it.
Frogs are very sensitive creatures and can adsorb chemicals and perfumes though their skin.

Again please let your son do some research on what kind of frog he wants and what it needs to be
happy and healthy.
and let it be a birthday present or late christmas gift.