Just to echo what others have said - whether you chose to send your DS to a group tuition centre or to an individual tutor, depends entirely on what setup would suit him best.
Some children thrive in a group (they enjoy a little bit of competition against their peers), some may find it a bit intimidating and yet others may just take a chance to 'hide' at the back of the room to avoid having to do too much (from what you're saying, the latter would not be a risk with your DS
) What suits your child best, only he and you are able to tell, so it is very difficult to advise anything.
As Tinkers pointed out, your son seems to be doing pretty well working with you at home already, so maybe you do not need a tutor for your DS anyway?
By all means, get one if you feel it would make a big difference to your DS's preparations, but don't think you have to hire a tutor just because most of other children you know (I'm guessing here) are being tutored. Good luck.