You should be able to find out some information using family history sites like ancestry.com - you need their names, and approximate year and place of birth, and you should be able to find birth, marriage and death certificates that you need. Once you know what you need, you can order the certificates from the General Register Office - www.gro.gov.uk
Definitely worth a call to the passport office, but anyone can order birth certificates and marriage certificates. It's just a question of having the data to obtain them (which surely you can get from your son's grandma if you are in contact).
So, total worst case, it's extra expense and delay. It shouldn't be a bar to getting a passport.
But as others have said, I bet there are ways round it if you ring up. Does your XP's mother have her certificate?
I didn't want to read your post and run, but hopefully someone will have more helpful info than me. The only thing I can suggest is that you try to speak to someone at the passport office. I'm sure they have had situations like this before. After all, what about people who are lone parents and who have never had the father of their children around?
Can anyone tell me how this works? I'm worrying a bit because I don't think I'm going to have enough information about my ex-partner's parents. (We were both born after 1982 so need the grandparents' details.)
I'm not in contact with him, but through his mum have asked for the relevant details, but he hasn't seen or spoken to his father in many years. Now the booklet says because my ex doesn't have a passport, I'm going to need birth and marriage certificates for his parents. I don't think this is going to be possible - what happens in this situation? I don't know if I'm even going to get complete information for this because he's been estranged for over 25 years.