First off he actually looks great. Size in BTA is hard to determine by the size of the oral disc as their behavior makes this far from a constant. Lighting, flow, placement and abundance of food all seem to cause the nem to stretch or suck against the rock, spread wide with stretched tentacles or remain compact with bubbles. Their is little information on what actually causes them to do which.
Health is best determined by whether the mouth closes or gapes, if the nem closes up or sags at night, whether the tentacles are "sticky" and whether or not the nem has the entirety of its tentacles (they begin to lose their tips if malnourished).
As for feeding it is typically not necessary as they can survive on light and filtering alone. However, they do grow faster with once or twice a week feedings, but they should not be fed acclimating or their health appears to be fading. Nems have a one way digestive tract that is quite simple, but it makes it difficult for them to expel food at times after having eaten. If the food can't be expelled it can begin to decay in/on top of the nem causing it to stress.
I would wait at least a week before trying to feed your nem, a month would be best. Let it acclimate first.