Hi Didi,
First, congratulations on your genuine care for your boyfriend ! You seem to be a kind and understanding person, and this is what it takes if your boyfriend has POIS.
When you feel that he is easily irritated, it may be better to let him some space, until his POIS symptoms are less severe. Usually, the first and second day of POIS are the worst, and then it slowly gets better.
That is great if he finds something that shortens the duration of his POIS attacks, and makes the symptoms less severe. You will see if 5-HTP continues to be beneficial or not.
Tell him also that a healthy diet is also helping. It is better to stay away from refined sugars like soda and chocolate, and not eat junk food. Instead, he should include more fruits and vegetables in his diet. Some members also avoid gluten, and some others avoid dairies or alcohol, but it depends on personal digestive intolerances.
There are some simple methods members have found to be useful to avoid POIS. For example, taking niacin before sex, and waiting for the flush to occur, and having sex and ejaculating after that. It does not work for everyone, but some members have no symptoms when they do that. It is one of the methods that he can try. You can find other methods listed here:
https://poiscenter.com/forums/index.php?topic=2338.msg19448#msg19448 .
I am in the same relationship for over 30 years now, so it is possible to adapt to POIS. But you need compassion and understanding.
For work, it can be a problem when he has severe symptoms, so he must find something that at least reduces his symptoms so that he can work despite having POIS. At the least, he should abstain from sex when working the day after.
Do not hesitate to come back and ask questions !