Vitamin supplements are unnecessary if they get unlimited hay, a good quality pellet feed and a variety of fresh veggies. Sprays just make the food taste really yucky, so we get even less nutrients than usual if you use it. Drops in the water are just as bad, so also avoid these.
When you say not eating, do you mean eating nothing at all? How long has this been going on? Piggies not eating is a bad sign. Maybe his teeth hurt him when he chews - he could have overgrown incisors or molars, or malocclusion, both very painful conditions. Does he refuse all food? Have you visited the Vet to get an opinion? Not eating isn't a sign of deficiency, it is a sign of illness, so you really should have him checked out by a professional before trying treatments.
x Lady Tuppence x