Kasia, DS was like that - would rather go hungry than take the risk of irritated lips/mouth, constant vomiting and the knowledge that those symptoms might develop into something much worse (anaphylaxis). It’s pretty logical, when you think about it from their point of view I was worried sick at the time, as you are, and felt that I was failing him. What got us through it was taking the pressure right off. My anxiety was reinforcing DS’s anxiety, so I had to force myself to calm right down.
I just focussed on making sure he got enough calories, vitamins and minerals, and didn’t worry about the form he got them in. So pasta twice a day if that’s all he would eat for lunch and dinner, but healthy fats and vitamins via the sauces (hide vegetables and chicken/mince by blending into a tomato-based sauce). It’s It great that your son will eat fruit - lots of that, preferably fresh but also in fresh smoothies. Unlike your DS, he could eat wheat and dairy but not fish, eggs, tree nuts and some pulses (chickpea, peanut, green peas) so making sure he got enough protein was tricky. Chicken featured heavily in his diet but it couldn’t be on the bone (he reacted) so lean fillets only. A chewy multi-vitamin every day. Safe biscuits and sweets, if we could find or make them, and he wanted them - I didn’t limit them. Variety was not so important as adequate nutrition, and peace of mind for DS.
It took about 2-3 years, I think. DS got older and understood his allergies better - didn’t feel so helpless and overwhelmed. Compared to children without food allergies he still has a restricted and rather repetitive diet, but that’s allergies for you. He got into sport, so realised that he needs food. He’s now grown up - tall, fit and strong.
I do understand your worry about social situations. It is sad when they can’t eat what their friends are eating, and awkward that they always need additional precautions. But that’s becoming less of a deal these days, as people (slowly) become better informed about allergies. What I found was that DS’s friends were often more aware than their parents - I always called/emailed parents before playdates/parties to explain about allergies and offer to send DS with food that he could eat, and the patents would tell me they already knew as their kids had warned them that DS couldn’t eat X, Y and Z so they needed to get something nice that he could eat! Not always, I admit, and I did worry that DS maybe didn’t get invited to as many things as he would have if there had not been the allergies to cater for. It is a worry for parents of non-allergic kids who fear they may end up inadvertently causing serious illness. It was necessary to do a big reassurance number on them too - email/call in advance, clear instructions about safe foods (and not to worry if he “isn’t hungry”), where the antihistamines/epipen were and when to use them, my or his dad’s mobile number and that we’d be round the corner in case of any concern. Unfortunately it is necessary to be a bit of a helicopter parent, but the main thing is not to let DS know too much about that. Despite all of that, he would often come home from play dates/parties not having eaten. We just fed him extra at home with what he would eat. Residential trips with school were more of the same. I bombarded the teachers and centre with info, sent DS with a load of snacks that he would eat and reconciled myself to the fact that he would eat the same thing every day or not very much. I made sure that his favourite/familiar foods were available in quantity when he got home.
Sorry this is a long post, but I really understand where you’re coming from and I feel for you. I think that people with several serious food allergies can never have the same relationship with food that the rest of us do - I don’t think DS will ever see food as a great pleasure. Looking on the bright side, however, he’s unlikely to turn to it for emotional support, which is probably a good thing.