Talk to your instructor (in private). Express your concerns. You don't say how old "Tom" is, but if his parents are involved in some way, perhaps your instructor could speak to them.
Keep in mind, though, that your instructor may already know and may already be doing what he/she can. Don't assume that, but understand that might be the case. I'm thinking of one martial arts instructor who runs his own school and he's aware of what a lot of his students are going through. Fights with alcoholism, substance abuse, sexual identity, past criminal activity, and much more. Sometimes he's in a position to help. Sometimes not. But that's who you should discuss it with first.
Hope that helps.
Don't leave him alone! 10% of anorexics are male. Anorexia Nervosa is what it's called. You should get him to see a counselor and into therapy. Anorexia is a dangerous sickness and is one of the three most deadly mental illnesses. Get him help and be there for him the best you can. If it's only a few months, it may not be too late. I suffered for 6 years with this illness and it has major effects on me still today. Talk to him and talk to someone who can get him medical help. He will come back to his happiness when he is better. Don't give up on him!
? is right. He needs to get medical help. He has either anorexia or bulimia. Both are eating disorders with devastating health consequences due to malnutrition and vitamin deficiencies. Untreated the sufferer eventually dies.
Talk him into seeing a doctor. The doctor will get him the help necessary. Don't leave him though. He needs a friend right now more than ever.
Everyone else gave great advice but something like this can be challenging to bring up and tell a friend and training partner. If you don't think you can do it right or if you don't think he'll listen to you talk to your coach and get him to talk to Tom about it. Good luck hope Tom gets the help he needs.
Worry about yourself
Let's call him Tom, because it's nothing like his real name. Anyway, a few months ago someone at his school said he was like that guy who's now ruling North Korea, because he's "Asian, fat and stupid". After that he started losing a lot of weight, and he was thin to begin with. He's probably lost like 20 kg by now, I'm scared for him. Last night at training he couldn't even do the warm up and almost fainted, his arms are like sticks and his pants fell down last week because they no longer fit due to the amount of weight he lost. I don't get it, he refuses to eat and he's really thin. I think there is a name for this but I don't know what it is. I'm really worried about him, he used to be so happy and now it's like he has no soul. What should I do? Just leave him?
