Advice please on farthers maternity leave etc

So my partner and a lad he works with are both going to be dads at roughly the same time.

My partner and him are not allowed time off at the same time as only three men who do that job on the entire site.

This other lad doesn’t book days off in advance sometimes just rings up on the day to say he has family issues.

I am worried my partner won’t be able to come to scans with me and have the time off when baby comes due to his other work colleague.

I am beyond frustrated and stressed because he couldn’t come to one appointment already because his work colleague went to doctors with his girlfriend about confirming their pregnancy.