Last weekend my boyfriend was out at the pub and got pulled up by a police man while walking along the street and being silly and nearly got fined for drunk and disorderly behaviour. The fine was meant to be $150 and he managed to talk his way out of it by being polite, apologising and explaining that it was his first night out since turning 18 and he was just having a bit of fun. However today, 9 days later, he was called by the police man who said he'd had a change of heart and now wants to fine him.
Is this even allowed?
It seems really weird that a police man could just turn around over a week later and decide to fine someone after saying he would drop the fine.

Laws very by location, since you used the term PUB I will assume you are not in the US ?

But I will base some things on laws in general for "most areas"

First the police officer does not fine. The police officer will write them a ticket or charge them with something. That will force the person to go to court where the court and/or the judge will fine them. In cases where they pay the fine without going to court, the court is the one that sets the fines.

Yes, a officer can change his mind, they may have found this person has a habit of this, was lying about his birthday or more. Most crimes have a SOL time, and a person can be charged with that crime anytime during that. Depending on location and type of crime, it can be a few months to many years. While this is very uncommon, it is possible.

A couple possible, he started bragging around town how he talked the officer out of it, and it upset the officer.

Another, this was not really an officer, I don't know of them calling you up, they would have just come by or mailed you a new citation. It sounds more like some of his friends messing with him now.