Hi, dont worry about the call request, that is an automated response - I think we will be able to resolve this on the chat.
Contact and maintenance are completely separate issues and whilst he may be withholding this as a form of "punishment" as such for not having contact with his child (or knowing your whereabouts) that is something that he needs to deal with separately - if he hasn't already tried.
If he has made an agreement to pay, then he must pay. If the agreement to pay was made under the Child Maintenance scheme you should contact them directly and they can take action against him. There are a number of things that they can do directly to force him to make the payment.
In terms of the behaviour being categorised as financial abuse - yes, it is. There is currently a Domestic Abuse Bill going through parliament creating a statutory definition of domestic abuse, which is to emphasise that domestic abuse is not just physical violence, but can also be emotional, coercive or controlling, and economic abuse. It is not yet been ratified as law, but it could be useful to refer to if legal proceedings ultimately were needed.
I hope this assists. If you have any other questions, please let me know.