Dear K.,
Choices about diet are a strange thing.
I, personally, was practically a vegan from the time I was a very small child. I came from a family of hunters and fisherman and I just could not bear the thought of eating something that had to die for me to eat. It broke my heart. My mother hated me. I wasn't really a typical picky eater....I would eat any vegetable or fruit...I just couldn't deal with eating the flesh of another living being. I did eat cheese and yogurt. But, no eggs, I wouldn't put milk on my cereal, no fish, no chicken...nothing. It almost seemed like my mom deliberately made things that I wouldn't eat. Liver. Scrambled eggs with oysters. Deer meat. Chicken hearts. Beef tongue. I would have been happy just going to bed hungry, but that was not an option. She tried to literally force me to eat it and I ended up vomitting and I swore I would never do that to my kids.
To make a long story short, I was a vegetarian all my kids' lives. But, I let them taste everything. I did not transfer my food beliefs onto them. My kids would eat absolutely anything. And liked it. I cooked things for them that they and my husband would eat and let them have at it. My preferences didn't need to dictate what they like. And they like everything.
My 22 year old daughter went through a vegan phase a while ago. There was really nothing I could say about it. Someone had her convinced that her allergies were caused by not having a vegan diet and she listened. As a result, she just ended up hungry all the time. No cheese, no yogurt, no nothing. She's getting over it. It's too hard to eat that way if you are trying to only eat pasta with no eggs, no salad with dairy based dressing, no biscuits and gravy....no mashed potatoes if there is milk in it or butter on toast.
Just so you know...a few years back...I was at a friends house and they cooked some steaks. It smelled so good. They asked me to just take one bite. Now I eat fish, chicken, steak, eggs, all kinds of things I wouldn't eat before. Not every day, in fact my 13 year old son cooks all the meat that we eat because I have no experience cooking it and I tend to cremate everything.
Your son is an adult now and you can't control what he will or won't eat. I know about the worrying part, believe me. That's why even if I wouldn't eat a certain thing, I let my children have it.
Do some research online. There is nothing wrong with being a "vegan" but it is super expensive to be doing it as a fad. My daughter found that out the hard way and she is back to eating what she always ate before. Zero junk food. The only time my kids ever got to go to a drive through was for ice cream. Or the fake version of it.
Like I said, it was peer pressure with my daughter but none of her friends stuck to it. Not there's anything wrong with being organic and vegan. But, she had about enough of fricking starving and that was the end of that.
Just try not to worry too much. My daughter is away from me too and I miss her and worry....but we try to raise them to make their own decisions and we have to let them do it. With my daughter, I found the less I tried to keep her close and check her diet and where she was....the better. They will never understand what a parent goes through until they have a kid of their own and for now that's one thing we don't want to speed up if they aren't ready.
Hang in there. Your son will be fine. No matter what he eats. I am a living example.
Best wishes!