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.
thanks for the advice re vitamins.
Just to bring you up to speed with Dr - he had his teeth cut back a few weeks ago and after the surgery he developed anorexia. He has been monitored very closely by our vet who gave him intense recoevery food and a vitamin B12 injection to enhance appetite (which worked amazingly well). He is now eating well, although is still far too skinny. I think it's just a long haul. I am taking him to be weighed next week. He certainly seems much chirpier now!