t may be that the son has some form of autism, as yet undiagnosed
Why on earth would you suspect he has autism at all? Where did that come from?
One of my, now adult, son's autism made him scared of all foods except those of a certain colour. However, he was never rude enough to ask for different food at anybody else's house, and I was never rude enough to demand just foods I knew he felt safe with. I would generally inform the host, to defuse any possible awkwardness. But he would eat what he could off the plate. A PP stated about putting a little of some food the child does not eat on the plate every meal. That is what we did, and it slowly began to work. I believe eating out with foods on his plate that he refused to eat, benefitted him, because that food would gradually become more familiar and he would be more likely to eat it. My experience leads me to believe that as parents we can do more harm than good by continually giving the children the limited food options they will accept. The middle ground is most certainly the way, and I would never, for instance have cooked sausage and chips at someone else's house.
Just as an aside I now foster children, and of the dozens and dozens of children I have looked after I have NEVER come across a child who did not enjoy at least some part of a roast dinner.
Also as an adult myself, and as the children grow older, food allergies aside they would never, ever refuse to eat any part of a meal cooked for them by somebody else. That is beyond rude and I would never stand for it.