I think all your documents supplied above is enough...as long as you are the only parent responsible for the day to day basis of your sons activities you should be fine...
Anyway, here's the list of certifications and letters I attached with my daughters application to go with me as my dependent when i applied the visa:
- certification from school principal (with dry seal)
- certification from local parish (with dry seal) in which we attended mass every sunday
- certification from school in which she attended her summer advance classes
- letter from my daughters GP (pediatrician/pulmonologist)
- letter from coach/trainor (sports)
- all original report cards (school) and achievement cards in which all my signature is reflected every grading period.
- bank account in which i am the one in trust because she is minor (8years old)
- health insurance
.... I also provided parental consent (notarized) from my ex in which stated that we plan to settle in the uk and that he is permitting my daughter to settle here...in your case you will not need this as you said you were abandoned since your child was 3months old.
Goodluck!!!