I don't get why people are saying he can't have ammo.
He can't buy handgun ammo until age 21, and he can't buy a handgun from an FFL in Florida until age 21.
He can certainly OWN a handgun and its ammo at age 18 plus in Florida; I did a couple decades or so ago. (Unless the law has become stricter, which I don't think it has.) I bought a GP100 in a private sale at age 20, and my mother used to buy .357 or .38 for me to keep it fed.
Not a lawyer, though - and it's always a good idea to get legal advice from a lawyer where you live.
(But he can own both the gun and ammo, and he should be able to keep it loaded, in his closed glove box.)
No on-the-body CCW until age 21, in FL, since FL requires one to be 21 to get the permit.