Oh ... hmm ... my immediate reaction when I was reading your first paragraph was that he wasn't interested

Then reading on, he has me totally confused????
To my mind, if you fancy someone AND feel a spark (which I vigorously add - you ALWAYS feel from the beginning) you date them and see what happens. No-one knows what will happen in the future and the point of dating is to get to know each other and if it's meant to be long term, it will be. It's not, to my mind, something that you can plan right from the outset but that's just my opinion.
I think the reason he has brought this up now is a kind of "get out clause" tbh (sorry - again it's only my opinion) he realises that he likes you but knows that you're not the one for him so he's letting you know - which in one way is a good thing but if a guy said that to me .. it would be a deal breaker. I've been here .... dated a guy for 18 months, we got on well and had a good time together but I always knew he wasn't the one for me. I had been honest with him about this from the start. When I ended it, he was devastated and I felt like a complete b*tch (even though he'd always known how I felt) and I vowed I wouldn't get in that position again either on mine or their part.
I guess it's up to you to decide ... as you said your 2 long term partners said the same and they lasted quite a while BUT ultimately they ended for whatever reason. If you can keep your heart to one side and not fall for the guy ... then carry on, see where it goes otherwise if I weree in your shoes, I would be moving on I'm afraid.
Jx
Edited to add : I have a friend in this situation .. she has been dating her OH for just over 2 years ... twice he has had indepth conversations about how she's not the one but he enjoys their company and likes dating her .... unfortunately, along the line she fell in love with him and each time now when she broaches "where the relationship is going" he responds with .. I have always been honest with you .. I like things as they are. There's nothing she can say now and her only option ultimately will be to end it but at the moment, she's sticking to treading water
x