I would either fold or just complete, unless this BB is folding a substantial amount of the time. I also don't think I would bet the turn again, but given that you did, I think the river is a great card, and justifies another bet. If he had the flush draw you would have been semi-bluff raised somewhere. Even if he is slowplaying, that 3rd flush card gives him less incentive to raise you, and since it is HU, I would call even if raised (I also would really want to know what he could possibly have to make him take a line like this if he is indeed TAG).