What is wrong with saying "My son loves his lego."?
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My guess, that it is due to how awkward sounding it is to those that don't use it. When speaking of nouns it is generally a base level rule to add an 's' to pluralize something. And though there are exceptions. They are not part of the common mans vocabulary in many English speaking areas. However I would guess you know that as well as I so that may have been a rhetorical question.
So... "My son loves his lego." would appear to also be incorrect as The LEGO group themselves use it as part of a noun-phrase with very rare exception. Examples taken directly from there homepage: LEGOLAND, LEGO designers, LEGO augmented reality, LEGO creations, LEGO education. When referring to multiple components they say LEGO bricks, parts, or pieces in all the examples that I could find. In fact the only places I can find were they say 'LEGO' they are referring to the company and it is of course not
his LEGO.
Basically if you just insist on being grammatically correct 'LEGO' is the non-countable (having no plural form) part of a noun phrase that can be countable.
Considering all that and that language is an ever evolving beast, this seems like a form of political jousting for your preferred evolutionary path... and now that I have come to that conclusion if I hadn't bothered to type all this I wouldn't bother to hit submit.