RiverTam -
This is a child who 'explodes a bit when pushed'. This is not an isolated incident.
There is always a better way of dealing with a situation than shouting and using inanimate objects inappropriately and whatever other ways he 'explodes'.
He needs to be taught to recognise when he is getting angry, and to recognise that behind that there is fear or anxiety or the need to control, frustration about breakdown or ineffectiveness of communication, anxiety/panic - feeling cornered/panic, feeling misunderstood/panic.
He can start with learning to recognise physical sensations of stress as his buttons are pressed, and to breathe deeply and consciously relax tensed muscles. Mentally counting backwards and focusing on the breathing are techniques to defuse his adrenaline flood. Parents could label this 'taking control'.
At home he needs phrases or hand gestures that his family understands to communicate that he recognises he is getting overwhelmed, and he needs a safe/quiet place to go to where he can breathe and relax and try to figure out what just happened and where to go from there. He is not too young to do this.
Away from home - in this instance he needed phrases that he has been coached in, and assurance from his parents that he can turn down food or other pushiness from adults and nobody will really mind as long as he remains cool and calm. He might feel empowered or more confident if were to be equipped with problem solving techniques and effective communication strategies, and if he were to be trained not to see conflict as a situation where he must win or the other person will. In any event, he will feel more confident that situations will not get out of control.
He needs to learn to distinguish when people are not getting at him personally, even though he is the one feeling intensely annoyed. A heads up about the possible motivations of other people when they address him can be delivered by the parents. Something along the lines of, 'The older woman probably does this to everyone, not just you'. He is not too young to understand that pushing food was all about her need to be the provider or to feel that her contribution was not going to go to waste. He needed a way to let her know that he was not going to fill that need.
This is where acknowledging her afternoon's work comes in - 'They look great Mrs X, but I am full. Thank you all the same.' Or, 'Sorry, Mrs X, I am afraid I can't fit one more bite.' Or, 'Thank you so much for your kindness, but I am completely full'. Spoken with a smile. Repeated requests - 'No, really, but thank you'. Then just shake head sadly from then on.
There were two people here who did not know how to solve a problem or defuse the situation. (I accept that the older woman should not have caused the issue in the first place, but that is not a problem anyone here can help with). It's important to teach children that most conflicts or breakdowns in communication are opportunities for problem solving. He solved the communication problem in a way that was certainly effective, but not advisable or useful for future reference. This should be pointed out to him.
The DS should have used the regretful smiles, phrases declining the offer that included appreciation of the effort and finished product, followed by head shaking. He and his parents need to role play and brainstorm, asking for examples from him of 'problem solving' or 'good communication' and chipping in some ideas themselves. There may be more buttons the OP has noticed, and they could rehearse a few of those situations too.
Sometimes a child who is normally quiet and polite (as the DS is) is experiencing feelings of frustration just under the surface that he is not dealing with. Quietness and politeness do not always indicate a well adjusted child. Often a quiet, polite child can experience high levels of anxiety just under the surface.
Empowerment and encouragement to take charge of situations themselves are encessary - parents really should assure children that they do not owe compliance to all adults purely because they are adults. They need to be shown how to dodge the pushy ones (and those who are devious, manipulative or deviant). Training them to be 'good' is not enough. They have to have a sense that 99% of situations are not the end of the world and that they can solve the problems they present.
Social interaction shouldn't be a huge, bewildering mystery. Children need to be taught how to read situations and to deal appropriately with them. They need to be encouraged to dissect situations with their parents - parents need to listen to the children's interpretations and to suggest what might have been going on. This is part of guiding children towards maturity.