Immunotherapy - Please share your experience

Hello everyone!  I hope you're all doing well.  I'm here because I'm hoping some of you might be able to share your experience with immunotherapy as a treatment option.  My father just rounded his 3rd year anniversary since his stage 4 lung cancer diagnosis, and I can't believe we're even here!  On August 2nd he will be 62 and it is also the 3 year anniversary of his first chemo treatment.  He's had a lot of ups and downs in those 3 years (mostly downs), and has largely felt lousy.  He finally decided to throw in the towel on chemo in the last couple months, but not on life.  He was tired of feeling crappy, and as someone who suffers other pains on a daily basis, I totally get it.  His oncologist innitially wanted to try some chemo pill after his decision to stop traditional treatment, but couldn't get insurance to cover it, so it's no longer an option.  Since he has stopped chemo, he has been feeling relatively well, though still very weak.  He is getting his appetite back, which is refreshing, and overall is just feeling better than he has in a long time.  His oncologist thinks this is a prime time to try fighting his cancer with immune boosting treatments.  I mostly have received second hand info from my mom, who is getting second hand info from my dad, so I only know a little bit about this new treatment option.  My biggest concern is that I'm told that while ramping up the immune system, there is a very real chance that it could bring any low lying illnesses out. Of course there are other side effects as well, but this is the most concerning.  I have this overwhelming fear that once my dad starts this treatment he's going to go right back to feeling like crap and his condition will just plummet after that.  If that's what happens, well... I guess I'll be grateful that he at least had a little bit of time where he felt less crappy for a bit... but it will still be heartbreaking to see his good streak end so soon.  If anyone could share their experience with immunotherapy, I'd love to hear your stories.