My son is 17 months now, but recently, he has been asking for "nan-nan" the whole day through! In the mornings when he awakens at about 4:30 or 5:30am, he would drink non-stop till about 7:30am!
He eats on average 3 solids meals and 2 snacks a day. In addition, he nurses about 5-6 times. So, I don't think it's from hunger that he wants to nurse.
Could it be he's still teething? He's got 16 teeth all in, isn't it a bit early to have his full 20??