Yeah, this. With the added caveat of the "undiagnosed" possibility above - if he's poly, he kinda oughta say so up front, if he knows. If he doesn't know, then that's a stumbling block, not so much in the whoa red flag sense, but in the umm you have an issue you should figure out sense. If he knows and isn't saying, that's a bit of an honesty and openness problem.

'Course, it could be not-poly at all - he could just be really friendly and easygoing and ... not so good at making long term commitments. Which is not a bad thing, but if LW is looking for a life partner, then it's not compatible.