The OP came on here for legal advice. Instead she gets a lot of people treating this as AIBU, some making assumptions about how much the boy earns and some making incorrect assertions about the law.
The legal position is that it doesn't matter how much the boy earns - whether it is just pocket money as a number on here are insisting, or whether he is in fact earning a good wage. The fact that the father has been off work for 20 months after breaking a bone in his neck is also not relevant to whether maintenance is payable now.
What matters is the education the boy is receiving. It is classed as "approved education" if it is an average of more than 12 hours per week of supervised study or course-related work experience, is not paid for by his employer and is one of the following:
- A levels or similar
- T levels
- NVQ or other vocational qualifications up to level 3
- a traineeship (assuming the OP is in England)
- Scottish Highers
If it meets the above requirements, it is "approved education" and child maintenance is due. If it does not, no child maintenance is due. That is the correct legal position.
If no maintenance is due he can, of course, choose to pay maintenance to his son's mother but she cannot force him to do so. He can, if he prefers, pay money direct to his son. But he does not legally have to pay anything.
Of course, if maintenance is due it must be paid to the mother.