As you know, as we age, things change, and we have to do the best we can, since mostly the changes cannot be reversed. You could go to a urologist and see what he/she says about this. I have heard of prostate massage performed that might be helpful, even within the urologist office setting. My take is to continue to stay as healthy overall as you can by having a proper amount of sleep, eating proper foods, getting enough daily water, avoiding all tobacco products, getting a good amount of exercise, continue to engage in sexual activities, and so on.
I don't have much experience with this yet. Guess I will as I age. But I am a supplement hound. The two supplements I know off the top of my head that are thought to increase volume are Maca, and Zinc. I dont know your medical history, so I dont know if these are right for you. But those are two supps to research, speak to your Dr about, and if you are both happy, maybe experiment with. If you go the Zinc route, get a quality Chelated zinc. Stay away from the stuff on super market shelves. The body doesnt absorb it well.