I had a tutor in maths from Year 9 so hopefully I can help. A lot of it depends on your son. For me, in year 8, 3 hours would have been too much but of course that varies from child to child. A year 8 shouldn't need THAT much tutoring unless he is having big problems at school and really does need a lot of help. If he is doing well at school and the tutoring is to further advance his ability rather then bring him up to scratch then 3 hours is too much (for year 8). For one subject, most HSC students wouldn't have more than 2-3 hours a week.
$10 is extremely cheap. Like very very cheap. Standard for 1st year uni students who did very well at school is about $40/hr and then go up to $120/hr for ex-teachers/HSC markers. Looking at group classes would be your best option because they are less expensive and you can have a good teacher looking after a couple of kids (Have a look on the internet for these). Keep in mind $13/hr is still not a lot.
I need to know the subject he's being tutored in before I can tell you what to look for in a tutor of that type so let me know