What are my options for getting financial support?

My son’s father has not lived with us for 6 years. In that time he gave us £700 for a car as he took the family car and just over a year ago we agreed for him to pay us £50 A MONTH towards the up keep of our son. This lasted approximately 4 months. He or rather his mother had given us some money here and there for school shoes etc but over all it has been approximately at the very most about £100 a year. Not always and not regular. I would like to know what I can do. Father has 2 other children, my son feels left out, physically seeing his father and going without through no financial support. Please tell me what I can do.