Hello B.,
I know your problem as well. My daughter doesn't like vegetables (exept for carrots), fruits (exept for bananas), salads, noodles, mushrooms, meat or rice. She drinks only water and milk, no juice, tea or anything else.
She's a rather small, thin girl and we have had times, she drived me crazy!
BUT: she's never ill! She's now 32 month old and she was ill for two days with gastroenteritis - she puked one night and that was it.
That's the first thing which helped me not to overreacted.
The other: most children eat - maybe little portions, maybe not the food their mother would love to see (me for example: I WISH SO MUCH that my daughter would eat fruits!), but they eat, often more than their mothers have got a notion. Try to note for some days how much and what food your son has eaten. Maybe you will be surprised (I was).
I wouldn't cook different meals for diner/lunch. Maybe it's better to cook on one day a meal which he likes and on the next day something new. With this method I have had small achievements (but achievements). As we "started", may daughter only eats potatoes, carrots and bananas - now she likes a lot of soups (that maybe another possibility: my daughter often doesn't like a certain texture. So she doesn't eat broccoli "original", but she eats it as a soup!).
And, honestley, I never would give some pills or appetizers to her, never! I think, this may confused a lot more inside her little organism than it could help!
After all it's very important to me that she doesn't lose her "natural" feeling of appetite, she have to feel that she's hungry, I don't want to have any "fake-hunger-feelings".
If your son is healthy and if he doesn't lose weight continously (if this is the case consult a doctor!), please: don't worry to much.
Regards,
K.