I used to just imagine I was playing a computer game, running through the levels and shooting the enemies. I'd imagine I was playing it and get so absorbed in it that my erection would disappear soon enough. But I wouldn't always think of that, I'd change it around. I wouldn't think of anything sexual, or disgusting, but usually just fun, or mundane things that would change my thoughts totally.
I don't think you're doing much harm in thinking about this old man. If it works, it works. If you find yourself thinking about him, and your erection getting harder, then maybe it's time to change to a different thought
