My DS is also 13 mo. We have always nursed on demand, not on a schedule. DS "asks" to nurse using sign language and is pretty good at asking whenever he needs a little mommy milk. I've been working on teaching DS the sign for "food / eat" which he gets right most of the time.
Recently, when DS signs "milk", I ask him: "Do you want "food" (with the sign) or "milk" (also signing)?"
Often he'll respond with the sign for "food" and I'll give him a snack. Sometimes the snack is what he really wanted and he'll be satisfied, other times he'll finish the snack and then sign "milk" so we'll nurse also.