I think i have your answers:
1. You and ex made verbal agreement that child stay with him.
At that time ex didn't want support. All verbal.
2. Verbal isn't worth the paper it is written on. LOL
3. You have a court order that never was modified for 100 per
week, therefore, you could go back to court and file arrears
which would be legal, but not fair as he did support him that
amount of time.
4. He is currently only paying you 50 a week, He is in contempt of the court order as he never got it modified in court. So, I would bring him back to court, and go by his and your income and have the judge determine how much support you are legally entitled to.
I hope I helped.