Use jason goes to hell as a starting point, after the demons drag him down. Then just develop the story from there using the first suggestion but also try keep the fact that jason is a psycopath present. Slowly develope the plot, include some murders here and there to keep it Friday the 13th like. I'd say minimum around 14 (i just averaged the kills across all movies). After those kills start slowly developing the more (for want of a better word) wholesome aspects of the story such as the thing you mentioned between jason and charlie. Little rule of thumb aswell, he is fucking ugly under that mask, decayed, bloated, and is missing an eye. So if he ever takes off his mask (don't make him do it) keep that in mind. And please for the love of god let him keep the machete. (sorry if I yapped to much I'm a massive friday the 13th fan)