None of your argument following your observation makes sense.
None of the substances you mentioned contains MSG. MSG is Glutamic Acid (amino acids). Dextrose, pectin and carrageenan are saccharides (sugars). Citric Acid is an acid with a different chemical makeup from amino acids.
How did you come to the conclusion that all “Natural Flavours” have MSG in them? How do you even know it’s MSG? You based your argument from the observation that if you don’t feed your son a particular brand of jam or a particular brand of dairy-free yogurt, your son’s cheeks don’t glow red. Do you mean that not cutting myself makes me not bleed? Colour me surprised.
You clearly stated that both foods contain pectin yet you never took your son to an allergist to see if it is actually pectin or something else that’s common in both the jam and the yogurt. This clearly looks like an allergic reaction and should be treated as such.
This article sounds like an anecdote of how a certain diet changed your son’s behaviour. Good for you and your son, but please don’t use bullshit science to back up your claims.