Have you chatted to him about why he has stopped eating?
I have had and still suffer slightly from bulimia on and off. My daughter stopped eating over christmas and I was in a complete flap about it.
With my dd I spoke to her and was told by her she didn't like the food I had in the house (it was christmas I had loads!!). I felt it was anxiety about going back to school (loves school socially but had a terrible report and was worried about the pressure) and it was a way of grabbing my attention and focusing her energies on food because its easy for her to control and focus on that than it is school (where she feels very out of control).
My eating disorder is often a habits brought on by a trigger and often a lack of control in other aspects of my life. Once something has triggered my ED I get into a cycle which is extremely hard to break.
So I decided that instead of buying things I felt dd would like I asked her to make a list of foods she likes (giving her control over the food).
She made a silly list with about 5 items with only chicken pie (Nothing else just chicken and gravy!) as the main meal! So we went out and filled the freezer with chicken pies. She knew I bought them specially for her so felt obliged to eat it.
I didn't make a fuss or talk about it I just served her things from her list and in addition tried to talk to her as much as I could about her issues and just listen and to give her as much control as I can over her life (I know thats difficult at that age).
I think a combination of breaking the cycle and giving her some control and positive attention rather than focusing all my attention on worrying did the trick and she is now introducing other items into her chicken pie diet and seems to have chilled a bit about it.
Another thing we found helpful was smoothies! She wasn't as fussed about drinking as she was about solid food so we have started buying innocent smoothies (she won't drink or even try my homemade ones, thinks Im trying to poison her !) and if you are careful and can get the ratio right you can blend a sneaky carrot in or some supplements without them noticing, I normally use my youngest as the Guinea pig and if it passes the test I pour it back in the carton and pop it in the fridge with my dd being none the wiser.
If you don't have any luck maybe its worth referring him to a ED unit. Its a good idea to get to the route of ED early before they develop further. I wish I had gone earlier.