I started on a rocker and I believe that he would have more fun on a rocker considering he has never gone before. No need to strap him up to a camber board and have him hate his first experience.
Just my two sense...
Either way, There are many boards out there for your price range. you could even probably find a good package deal!
I'd just go to your local board shop and look around with your dad and ask whoever works there what they think your proper size would be. It's basically all preference, but since your beginning I'd ask them what they think a good size for you would be and this would most likely be a good place to start.