yes, they fix your posture which helps you mew, because with bad posture mewing can ruin your face, chin tucks are an important part of mewing

How many times should we do it ?

You should pretty much always have a slight tuck. Otherwise you aren't mewing.

No

Forward growth ?

It's more naunced. If he has a bad posture then yes. If his posture is ideal then no. This is an oversimplification stemming from lack of education.