That "S" curve look is really common with boas. Both of my babies do it all the time and it never means anything. Seems to be just how they look at the world, lol...

As far as feeding, you should never be too conservative just as you should never overfeed either. Just choose an appropriate sized rat or two and feed every 10-14 days. As he gets a bit older once every 2 weeks will be fine. And you can certainly keep him on rats. 1-2 large to jumbo sized rats should do him for the rest of his life. If you were to switch to rabbits, it could instigate a growth spurt. But the rats are fine.

He will grow as a big as nature intended if he's properly fed - but that does not mean he'll turn into an overgrown monster. At 5 years old and being a male, he really shouldn't get much bigger than he is now - he looks perfect.