which is why I have never questioned a night feeding if he wakes. Should I wean him from that?
I would. Obviously if there is a weight gain issue or medical issue that's another matter and you should discuss it with your doctor but if he is healthy and gaining weight then there is no need for a 14 month old to have a night feed.
The lower centile lines on weight are normal so long as they are stable and LO is keeping to their line, there is no need to try to raise the centile line or work towards "average" because all of those centiles on the growth chart are within the average.
Taking milk in the night could have the opposite effect that you want at this age, he needs to take the vast majority of his calories and nutrition from solids meals, not milk, extra milk in the night could effect how much food he takes in the day and become a bit of a cycle. If anything must be offered in the night I'd just offer water. And make sure his meals are high fats and protein as well as all the veggies etc.
Just as a reminder, toddlers need 4 portions of dairy per day which could milk or dairy solids. A portion could be a half cup of milk or yoghurt, a 1oz portion of cheese (1" cube). When you think about those sizes it is not a huge amount. If he has a bottle or cup of milk any time in the day that could be 2 portions (say 200-250ml). And of course every toddler is different so it's just guidance.
It can be really hard to get a LO to take an extra nap but it could just do the trick. See how things go, you can keep this one in your tools box and pull it out if or when you feel it could be worth a shot.