Passport issue

My daughters step mother has a bit of a problem, they are due to go on holiday for a family wedding next year, she has a son with her ex who is not being so cooperative.

He is on the birth certificate, but she holds residency of her son

Looking for some advice, or if anyone who has been in this situation could let me know what they did to obtain a passport.

She is willing to show the ex details of holiday as her & my ex did with me & let me take a picture of the holiday details.

He is just being awkward