Oh goodness, eating out with relatives amd kids who have food issues is totally a pain if they don't get it. Who cares how a five year old eats? Honestly.
I would be inclined to be ordering chicken nuggets and chips when they go out to be honest! You can get specialist cutlery for kids who struggle to hold a knife and fork though, and if he can't at all, I would say same as pps, that it might be sensory or ability.
You can get cool digger cutlery (literally shaped like a digger lol) and glittery ones that might encourage him, and you can practice with stuff at playtime. Can he scoop dry rice etc? The novelty .might help.
On a side note, I always think its interesting when psychologists say it is just their stage, because I think they can forget how socially excluding for children it can be to not do certain things at the right age. I know that we want to try and teach ours it before he really starts on school dinners for example because otherwise other kids can be really mean. I'm of the opinion that some things can be done in the privacy of your own home, and if they insist on eating beans with their fingers at home to fulfil a need, so be it, but if at all possible, I also feel they should have the option and capacity to eat properly too it if they want to, and for that it requires that we put some conscious effort into teaching and rewarding that if it is something important to you (which it is for us!) if you see what I mean. How does he feel about it? Does he want to eat with a knife and fork and is struggling? Or is he just relishing the baby stage?
I would be keen to ask whether the professionals thought he had the capacity to do it, and if so, how long should it take him to acquire the skills. Pin them down to a specific time frame- alright, if it is OK when he is 5, how about 6? Should you send in another letter if he hasn't learned it by 7? Because if he si.ply cannot physically do it, he should have accommodations being made like special cutlery etc and real in roads being made to find out what is going on. if he does have the capacity and is choosing, then there needs to be a discussion with people (professionals, family, OH etc) about increasing expectations gradually to fit where you are all comfortable.
I say all this as the mother of a kid who sometimes won't even sit at tables in public at the moment, let alone eat. Let alone eat with cutlery! We do expect it at home though, and he is working towards goals, just very slowly. I would be more inclined to flip it and say the expectation is always attempting to use cutlery when at home eating wet food (so you can support and clean up mess) but with fingers outside the home for now because who needs the stress of making kid implement a new skill in front of an audience lol.