I guess the answer may be no but was just wondering what my son will sound like when his voice changes. If he has a powerful voice now, is it likely to be powerful when it changes? If he is already good with low notes is that more of an indication that he will be bass - his father and brother are bass. Well if sub bass existed then that would be his father! In general, if a boy treble has a good voice, will he have a good voice as an adult?

Yes and No.

Your son is likely to have similar voice types to other family members, but this is by no means hard and fast.

It is easier to predict is you come from a family of tenors than basses, and given the age of your other son, although he is currently a baritone, his voice will not be settled enough to be certain that he is a bass.

As far as quality, again not really. By singing through transition and monitoring where the voice is at each point then the chance is he'll keep control at each point.

Again, what are the vocal qualities of family members?

It is important he does not push the volume when his voice is changing and the golden rule is if it hurts - stop.

He likes to be heard, so controlling his volume will be the most tricky. I've read so many things about the do's and dont's with the changing voice but I won't know when his voice is changing or what to look out for.

Check out some of the Russian 'basso profundo' voices - really spectacular! (And apologies for the post.)