I never received my court date letter?

First, were you under 17 when charged? If not, this should not be a juvenile defense. Was there an address change? If you didn't get a notice,...

What legal action (besides child support) can the mother of my son take against me if she already has sole custody?

What is it that you want to do for your son?

Is the bill you received an educational invoice or a medical invoice?

If you must ask this question, you need to hire counsel.