However, the answer is fairly straightforward if the father and mother cannot agree about the child arrangements together. They should try mediation as a first port of call; he will need to attend a MIAM at the very least before applying to court. If mediation is unsuccessful or the other parent won’t engage, the mediator will sign his form and he will be able to apply to court for a child arrangements order. The C100 form lists exemptions for those who do not have to attend mediation - your son should check to see if he is eligible to skip the requirement.
The CAO is applied for on form C100, and the cost is £215 unless you are on a low income or benefits in which case you may be entitled to fee remission, see form EX160. The court will make a decision based on the best interests of the child after taking all the concerns raised by either party into consideration. The court is likely to want a s7 or s37 welfare report to find out what is going on and to make recommendations to the court as to what should happen. As SS are already involved, this will be a s37 report written by the social worker already involved in the case. If there was no SS input it would be a s7 report done by CAFCASS. There will also be police and SS background checks carried out initially by CAFCASS to produce a safeguarding letter for the first court hearing.
Find a mediator here: www.familymediationcouncil.org.uk
C100 form: https://assets.publishing.service.gov.uk/government/uploads/system/uploads/attachment_data/file/946762/c100-eng.pdf
EX160 form: https://assets.publishing.service.gov.uk/government/uploads/system/uploads/attachment_data/file/952045/ex160-eng.pdf
E160A Notes: https://assets.publishing.service.gov.uk/government/uploads/system/uploads/attachment_data/file/958038/ex160a-eng.pdf
If you have any queries about what I have written in response to your situation, please feel free to ask.