I think that your kiddo needs to sit down and write all of his feelings out. It's venting on paper.

Then he could either send it or hold it, just in case he wants to add stuff.

I don't think that bio dad will care or have a grand epiphany -- I think in true narcissist fashion he will play the victim and not understand why his kid feels the way they feel.

If you can afford therapy and your kid wants it, I would do it.

If you have a big brother or mentoring type program, sign him up.

It sucks for him and my heart breaks for kids that have living parents who can't be bothered to take care of their own kids. I'm sorry. Neither of you deserve the crap you're dealing with.