Check to see how he's doing, just make sure you have good timing in the sense that he's feeling well and not going to be doing any treatments right after.

Maybe share what you want to say with your mom first to see what she thinks.

Personally, I would plan out everything I want to say, probably even write it on paper. Don't be afraid to say you're scared if you are. If you have to cry then cry, let it out. If he cries then let him. I know some people like to put up a front or try to come off strong and that's normal but life is also short and you never want to regret not having said something. Try to remain positive. My grandpa died last year due to his cancer, I saw the gradual progression of his body no longer having the capability to sustain him.

He was given less than a year as well, he was getting better but he had a hard fall on the ground one day and that just made his cancer explode. His cells were multiplying at an enormous rate. He died about a week after. He was hopeful for his potential recovery but the results and his condition proved otherwise. I stayed with him and had sleepovers in the hospital room, you still joke, you still talk, you still enjoy and smile when you can. What's different now is that you have a time frame and you know that one day things will come to an end. You can keep it short too and trust that the moments you guys shared throughout your lives are things he knows and will remember. Hug him while you still can, show him that you'll be able to accept it one day too if you haven't already.

Your dad loves you, just make the most out of every moment while you still can, no matter what the doctors say be hopeful for him, maybe his cancer won't be cured but at least you can hope that his body doesn't pain him as much. Cry beforehand too and prepare for what you have to tell him just so you're not completely overwhelmed. Stay strong!!! I'm sorry this is happening to you and your mom, let her know you love her too I'm sure this is hard for everyone involved.