I am sorry you are having such a rough time at work. However, when you threatened to leave before, you should have just left work. You should not have accepted the boss's counter offer of more flexible working hours. The real problem i.e. his rudeness has not been solved.
If you are the only person in accounts, I can imagine it must be hard to keep all the work up to date and take your 19 days parental leave. You are entitled to this, but small firms don't really have the cover to facilitate the full family friendly workplace. I can understand your boss being annoyed at people looking for flexitime & taking maternity & parental leave. Even though everyone with children is entitled to all these concessions, it does lean on the side of the employee and the law does not take note of the employer needing extra cover while key people are on their leave.
As soon as you discovered your boss's attitudes towards families, you should have started looking for another job (pref. in a large organisation) which has the facilities to cope with additional leave/flexitime.
Your boss tends to want to 'have his cake and eat it'. His attitude suggests that he wanted an experienced person for the job, but didn't want the hassle of employees having a family & a life outside of work.
You said you are interviewing candidates for the position. If you chose a single candidate who is very assertive, they could suit the organisation & the manager better. Or else you could choose a candidate that does not have work at the moment. At least they are not leaving another job for this job. Please do not choose the nervous candidate, as this candidate may have a breakdown & may not be able to cope with your boss. I know you do not care about your boss (and rightly so) but think of the nervous candidate. What goes round comes round, and it is unfair to take a nervous candidate out of a job if they are completely unsuitable.