Good morning, thanks for your patience.
In short he has as much right to decide what his child does in his care as his wife does. He also has as much right to object to his son spending time with his wife's new partner as she does to his.
The family court won't prevent him from introducing a new partner to his son though, particularly if mum has already done this with her partner.
He should be having regular contact, including overnights, and what his son does in his time is up to him.
The good news is that the costs of this are fairly minimal, as far as court fees are concerned.
Mediation has to be the first point of call as it is a legal requirement prior to issuing an application in the family court.
You start this process by attending family mediation - see here for a link to find your local mediator:
Assessments are usually around £30-£40, and the mediator will advise as to how many sessions might be required. Mediation is a collaborative approach and allows you to talk through issues and concerns in a safe and protective environment.
Following that, if the mediation does not succeed in a successful agreement, you can apply to the family court for a child arrangements order (see below description), setting out the living arrangements and contact arrangements that you would like to see.
You would apply on a c100 form, here:
You send 3 copies of this form to the local family court centre.
Alternatively, you can apply online:
There will be a court hearing after you apply and a family court adviser will be there in attendance too (from CAFCASS).
The application costs £215.00, and you can represent yourself so you don't need to engage a lawyer. You can also possibly get a fee exemption if you are on low income/have low savings.
For this you would need to fill out a fee exemption form: http://www.gov.uk/get-help-with-court-fees
I hope that this answers your question?
Thank you for your enquiry today. I am happy to answer follow-up questions - please do get in touch with requests for extra information or further queries and I will do my best to help you.