Hi Cepheus,
Thank you for writing here, and showing that you care for your boyfriend. Men who have POIS need a spouse who has a good heart and will care for them even when they are in a POIS attack, and you seem to have this good heart
I, too, was transforming into an anxious and irritated madman when in POIS, so it was very hard on my relationship. My spouse has been very patient with me, but hopefully, I have found ways to lower my symptoms, so that it can be sustainable in the long run. I know you have very good intentions, but years of those severe symptoms will take a toll on your relationship. You are human, too. So, the best is to find a method that will help him have lower symptoms, and of shorter duration. I see you mention Fenugreek, which is helping many members. If it helps, keep it, and find something else to add to it. If you have not seen it yet, I suggest you take a look at the many methods found by members that have been proven efficient in reducing POIS. No method is efficient for every POIS sufferer, but when you find one that works for you, it is very valuable. See this list at
https://poiscenter.com/forums/index.php?topic=2338.msg19448#msg19448 . Some are really easy to try, like the method with niacin, or the one with OTC antihistamines. Anything that lowers the severity and symptoms of POIS is a plus. Some imply a prescription drug and call for medical supervision.
At the least, a very healthy diet is good to implement. No junk food, no artificial flavors or colors, avoid refined sugars, add many fruits and vegetables, organic if possible. This would help anybody, and help POISers as well. Some vitamin D, like 1000ui to 2000ui a day, is good for everybody and seems to be beneficial to many POISers. Vitamin D alone is not enough to be a POIS cure, but with time, it helps make it less severe, it helps with inflammation, it is not expensive and not dangerous ( if you respect the usual dosage like 2000ui a day - high daily doses must be taken only under medical supervision)
As Muon mentioned, you can also consult the POIS Doctors list, and see if there is one he can consult. There is no recognized treatment for now, but they do their best to help and sometimes have some success.
Another useful way to limit POIS symptoms, which I have been using for years, but that needs a lot of practice and perseverance, is to have sex or masturbate without ejaculating. in my case, this will not trigger POIS symptoms, but I had to learn to manage the energy that is not evacuated, I also had to get the hang of it, at first, it is easy to stop too late and have an unwanted release. There are techniques that help, and tantric literature can help in understanding the principles and mastering them, but it can only be learned over a long period of time, so this is not a short-term solution. It is better to first find a method to reduce the symptoms.
On the emotional side, the best you can do, and I feel you already do it, is to not get hurt or offended by anything he does or says when he is in a POIS attack. It is a bit like Dr Jekyll and Mr.Hide. In his normal self, he really loves you. But, in POIS state, I know it is hard to understand, there is no energy to adapt to anything or to calm down - our body and mind are a mess and our emotions are just one big open wound, and even if you touch it lightly, the reaction will be pain and anger. Much like a hurt animal who feels trapped in the corner. Anybody, anything is a threat, even you, his loving girlfriend. So he can say mean things, and his behavior may seem totally incomprehensible, and exactly the opposite of who he was just before. That is where you have to remind yourself that it is not his true self, and not react. Just say that you know he is in POIS attack, and that you will let him have the time to recover. So, the best you can do is to leave him space, and not react to his anger and lack of kindness.
Also, do not ask for anything that would require energy or seeing people ( all we want in POIS is to be alone and rest). If you live together, do not ask for household chores, or anything demanding ( yes, I know, it is not a normal situation...). Do not ask to go out, to see friends, to go hiking or do any sport ( maybe a car ride, with no stops anywhere, or something he can tolerate while in POIS) Just a silent and reassuring presence is great, and doing with him what he likes to do when he is in recuperation phase (watching videos ? ). Of course, if you find ways to shorten his recuperation phase, the relationship will be much more enjoyable for you and for him.
Get to know more about his POIS cycle. For example, if his symptoms last usually 7 days, you can expect that the first 3 days will be the worst, and the last four will be better, so easier to incorporate light activities in this part. But when in the really severe phase, you will have to accept that your boyfriend is "temporarily out of order".
I hope this will help. Feel free to ask any questions you have!