Ok, now I am on the computer, I can write my long post. I think there are similarities with my ds1. He was a fussy eater from about 9 months. I remember he'd eat everything when he was a baby and then started refusing various things as he got older. I 'gave in' (not wanting to make food a big deal) so he had a lot of fish fingers and chicken curry and rarely ate the same food as us. I remember distinctly a holiday when he was 1.5, and stressing a bit about food, taking the ingredients for chicken curry. And another holiday when he was 2.5 where I took loads of tuna and cous cous (had to be Ainsley spicey!). He had a fairly wide range of 'favourites' which has gradually narrowed over the years. I have noticed with my ds that he is a lot worse when he is stressed about things, his appetite totally diminishes. Is it possible your ds is stressed. Does he ever increase his appetite? My ds is definitely a lot better in the holidays and also after doing running about. Winter is hard as there is a lot less running about and I think his mood may be lower (monitoring that one).
During the January/ Feb term, I realised he had not been eating dinner at school for a long time. (He managed for the whole of Y2 which was a happy year for him, and even the first term of Y3, but apparently they changed the dinners plus he was having difficulties in his friendship group). I only realised this when I gave him a packed lunch instead and it was coming back virtually uneaten and also saw just how much weight he'd lost (at this point his pelvis was protruding from his back). School were great and came up with a plan with me. I give him fruit, sandwich, yoghurt drink, snack (eg. raisins, rice cake, crackers). He had the fruit at morning, sandwich and yoghurt drink at lunch and snack in the afternoon. The teacher got someone to monitor him. It made a huge difference. He has put a fair bit of weight on. I have also started this thing where we all have a 'meal of the week' where we get to choose a meal and pudding. I wanted my other kids to be involved as I don't feel it's fair on them for us to always eat only what he will eat. For the sake of this, I sometimes give him something else (he likes omelette).
He tends to sit in front of food for a good 10 minutes. I now ignore this. Often, once he's tasted it, he likes it, and eats it. It's a really fine balance between pushing him to expand his boundaries and not creating an issue. You just need a lot of wisdom there. Often, once he's started he does like it. But I also try and cater to other needs, eg. putting sauce in a bowl on the side, not mixing veg into food (has to be separate, sometimes also on a separate plate). He likes plain food mostly, plain pasta, plain rice.
I think an eating disorder is a mental health issue. It usually goes along with unhappiness, isolating self, depression. I don't think your ds would be happy and busy if he had an eating disorder. I think he has some food hang ups, which is different. How are his friendships? How is his anxiety? I was worried about my ds (and still am) as his is always triggered to be worse by emotional problems. I have been to the doctor, but she just said to monitor it for the moment. At least it is logged that I have concerns about his eating.
The one saving factor for my ds is that he has a good breakfast. His favourite is Greek yoghurt, fruit and granola. He has always loved this. I also make honey bran muffins sometimes and there is a Sri Lankan breakfast with rice flour and coconut that he eats. He also eats omelettes. But, if he is unhappy or stressed, he would even be able to eat these.
From what you've said, I would...
Offer him plenty of drinks after school, including half an hour before food. Do this yourself too. Then no drinks on the table, as others have said.
Get him to pick a menu a few times a week.
Try as much as you can not to get upset about it. Sometimes I have to leave the room. It is so sad to see my 5yo eating double what his brother eats at times.
Definitely no biscuit and milk when he's hungry. Why don't you come up with a list of some healthy things he can have when he's hungry eg. nuts, granola (loads of calories in that) or the snacks from school or more of his dinner?
Try and get into his head a bit and whether he is really all that happy. My ds can be busy on his own or with his siblings but lots of stuff happening at school.