It might not be one thing, it might be a combination of thing's. I will say this though, never dismiss name calling, no matter how mild or silly it may seem. NEVER tell a child to just ignore it or it will go away, never tell them to tough it out or get over it, even if phrased in a nice, calm way. it matter's, it hurt's and it sticks with you. I know, TRUST me I know. they won't want to talk about it, they won't want to tell the teacher's, they DO make it worse at times, they can't be around to protect you 24/7 and bully's will always find a way to get to you. at times telling an adult will only bring on more retribution from the bully's.
It suck's, it'a unfare but it's the way most kids who are bullied think and feel. they also won't want to worry you and will suffer in silence or make your life hell getting them to school. they'll fake being ill, make themselves sick at school, be disruptive to get detention (it's a safe way to avoid bullying) but then this is pretty much based on my own experience and my own experience was pretty violent. it was an extreme case but it started small and got worse, i was told to ignore it, or that it wasn't serious or not to be upset at a bit of mild name calling, everyone was doing it. it was a slippery downward slope to be honest.
I was dyslexic and it went undiagnosed for a long time and the other kid's picked up on it, I was disruptive in class and was pretty miserable when the other students started to pull away and progress when I couldn't. it hurt to know I was struggling. I hated putting in the effort after a while because t was never good enough. I hated I had to work extra hard and that it was more difficult for me to try and attempt to keep up.
It's hard to pin things like this down to just one thing as sometimes it's kind of like a knock on effect into other area's of our life.
The meals at school, have they changed? is the menu not the same as it was. is the food actually making him feel ill or tired or a combination of the two? maybe try him on a packed lunch, perhaps request he be allowed to eat on his own or be first in or last out of the lunch room so not as many people are around to encourage him to eat. it might be that he's become uncomfortable with eating in front off or around people. home is a safe place because you live and eat there every day. does he find it off putting or a bit difficult to eat out?
Like I said it may not be just one thing, it might be a combination of thing's.
I hope he starts to feel better, I've got my fingers crossed for him