Call me a cynical asshole, but....I think you need to be more harsh.
I've had a (very mild) stalker in the past. He found out where I worked and made sure he was always outside, he found out where I'd go out (which isn't brain surgery considering Melbourne has 2.5 decent gay bars) and he was always there ready to coincidentally brush up against me in the crowd...that kind of shit. I tried to be nice, because I could see that he wasn't trying to have any kind of power over me, he was latching on to me because for most of the time I knew him I'd been too nice to just tell him to get fucked. I know that his obsession issues are a reflection on him and his self-esteem, and because I felt sorry for him I let a lot of what he did go (he also pulled the same shit on a friend of mine).
Ultimately though...you don't owe this guy anything. And until you make it abundantly clear that you don't want anything to do with him, he will always harbour some hope that there's something between the two of you.
You said he's in his 60's so I'm gonna go on a limb and assume that he's probably not really in a state to be a physical threat to you--and while it's sad that you can't help him through his issues, it's not your responsibility to do that either.
Hopefully what you've said will work. If not, be as harsh to him as you need to be. And if that doesn't work, do as has been suggested and get some guys involved who look big and tough. And in future, if someone is giving you attention that isn't warranted....as much as it shows you've got a huge heart for humoring them...let them know it's unwarranted. It's one thing to be compassionate, but you should never let your peace of mind or safety come second to someone else's comfort.