my experience: my daughter had a cavity between her 2 front teeth when she was 3, the ped dentist said there're 2 ways to fill, one of them is a permanent filling but requires her to keep her mouth open and stay dry for 10 min (i forgot the exact time) but my daughter was scared and cried the whole time, so we change to the 2nd method for a more temporary filling that lasts a year or 2.
i was told that if the child gets cavities this young, even their permanent teeth are more prone to cavities, so it's better to fix it than leaving it, unless you know he'll be practising perfect dental hygeine every day from now on.
ps. our dentist doesnt recommend x-ray for children under 4