Quoting Cariad (SuperKink):" I asked him straight out if he was into me and he said he was attracted to me but not into me cos I'm ... [snip!] ... once a week if not more and I just think it's not really worth any crap happening even if he's not trying anything on KWIM?"
Like I said, I think it's approaching an inappropriate level based on what he's saying to you alone. It's not fair to anyone involved, especially not your significant other OR
your friend. If he has feelings for you they're not just going to go away if you guys are seeing and talking to one another. They will only grow stronger and cause issues later on down the road. This has happened so many times to so many people and this situation is more or less extremely predictable based on the information you've given: This dude is going to develop even stronger feelings for you and it will cause issues between you and your SO, plain and simple.
If I were you I would stop talking to him altogether. Besides, no friendship should be this complicated to begin with. The fact that you're having to figure out how often you should be talking to him is enough of a sign to me that there's already a problem here. You're posting about how you should talk to him 'x' number of days a week and such and that's just bizarre. No appropriate friendship should be that complex. You're already making excuses for his behavior too and that's not good either.
One more thing: If you're serious about your SO and you guys are getting married then you need to respect his wishes concerning your friend. If one day he asks you to stop seeing him so much then you should do just that. Think of how you would feel if the roles were reversed and your fiance was talking to some girl who admittedly has feelings for him. You probably wouldn't like it much. I'm all for people being able to have friends of the opposite sex and whatnot but if one of the parties involved admits to having feelings for someone then that "friendship" needs to be halted for a while at the very least. It's basic relationship 101. You just don't do that.