Mmm, I would up the prey size. Many on here go by weight of the prey (15% of the snakes body-weight? I think. Don't quote me.) But honestly, I find it far easier to choose a prey based on how thick around the snake is- the rat should leave a small, slightly noticeable lump after swallowed. He doesn't look too bad, weight wise, but It's hard to tell from a picture. I would probably feed him a weaned rat every 7 days. Just me.Others can chime in.
Try not to fixate on his age vs. his weight so much. All snakes grow at different rates, and as long as they continue to grow, there should be no problem. It is a good idea to start keeping a record of his weight in grams, though, so if he starts losing weight, you'll know. That can help early diagnosis in a lot of health concerns.
edit: Oh, and about the head this...yeah, I think that's bunk. I've never heard that. (Then again, I don't know everything. I may be wrong.) As long as he's getting enough to eat, he'll be good.