What can I do when my child's father does not keep to the arrangements for paying our son's maintenance by standing order,and I have to contact him every month and remind him,and end up getting the money late?

If the late or non-payment of child maintenance is in contradiction of an arrangement set in a divorce agreement or court judgment/decision,and is part of a clear pattern, rather than one or two isolated incidents, then you can make an appropriate claim, based on contempt of court. Within this, you can ask the court to fine the father for his omissions regarding child maintenance,and for breach of the agreeement or judgment/decision concerning child maintenance. This type of legal action on your part is designed to put the father back on track regarding his fulfillment of his financial obligation to support your mutual child.