Just like you did here. Best just to be honest and answer his questions to the best of your ability. Beating around the Bush and sugar-coating the problem doesn't do any good for anybody!
How do I tell my friends and boyfriend that my health is declining and I might have a serious illness? My doctors said it's likely I have a terminal illness developing based on my current health issues that have been worsening. My family already knows. I don't know how to explain it to other parties.
You don't, other than your Family and BF, it really isn't any one elses business unless youre looking for sympathy. You just need to live life like nothing I wrong here and move on and enjoy it while you can. Only the important people in your life need to know this so you can't be accused of with holding info But its not going to do any good for anyone else to know this except you want to be smothered by people feeling sorry for you which sometimes is worse than the suffering itself. So if they don't need to know, don't tell. Sorry to hear this and good luck
My advise is to just tell them .. Share with them what you know about your condition, reach out to them..and allow them to reach back. This is a time to band together with everyone you care about and who care about you .. This way you can help to find the emotional support you'll need, at the same time as offering an opportunity to allow your friends to BE friends, when you need them most. That is your legacy.
I am so sorry about your diagnosis, and truly hope your health improves. I say that from the bottom of my heart .. Prayers sent.