Wow, that is one very under weight snake. If your pinky rats are new born start there as they will be unable to harm him if it takes him a while to eat. Since he is so skinny, he may also be weaker than he should be. I personally would not feed him a prey item over 10 g, just in case of regurgitation (but some more experienced keepers may disagree). The most important thing is that he eats. If you can get him to eat a rat pinky, that's a great start. I would not want to rush him onto bigger prey immediately since his system may not be able to handle it right away. I'd also keep him on a every 5 day feeding schedule, and maybe drop a fecal sample off at a vet if you don't know where he came from.
A live rat pinky given at night with all the lights off and left overnight may tempt him to eat. Since the rat can't hurt him, toss it in and leave him be overnight. (this is assuming the pinkies are live...if they are frozen, get them warm and do the same thing, you can even put it right in front of his hide door so he does not have to travel far to find it).
He looks like a petsmart baby to me...they never feed correctly or frequently enough it seems. The pic also suggests that he might have retained eye caps, so make sure his humidity is spot on.