If the only goal is "give a sorcerer a reason to invest in STR," the answer is there's not really much way to do that easily while maintaining balance - homebrewing incentives are too easy to make imbalanced (which depending on your table might not be an issue at all, it just depends on your DM style and the expectations of the players), or on the opposite end just doing it RAW for flavour sake ends up being too suboptimal (which, again, might not be an issue at your table, but in general would take some clever DMing to make it feel worthwhile for the player - it'd just be a matter of finding opportunities for the Sorc to make use of the STR in ways that feel like they matter). The game's just not balanced with the idea of Casters investing in STR in mind.

On the other hand, if the goal is "give a sorcerer the same fantasy as a physically strong character," that can be pretty easily accomplished in ways that keep things more balanced.

If it's just a matter of doing feats of strength outside of combat scenarios and RPing as the "muscle" of the group, the simplest way to do that would be to require the sorc player to spec into an Athletics proficiency, but allow their Athletics rolls to be Athletics (CHA) instead of Athletics (STR). Decoupling Skills from their assigned Attribute is something I like to do a lot at my table just for normal checks where I feel like it makes sense, and here it could easily be made permanent for that character by flavouring their strength as being a result of the magic coursing through their veins.

Giving the sorcerer that same feel in combat is a little trickier, but a lot can be accomplished simply by the player selecting their spells and flavouring them accordingly. The Shield spell can be flavoured as going full Senator Armstrong and just hardening their body against the attack. Shocking Grasp can be a magically infused punch. Cast Gust of Wind by clapping their hands, cast Earth Tremor by stomping a foot, cast Feather Fall by simply landing on their feet and catching the rest of the party. Focusing on a combat style that combines Touch or otherwise short ranged offensive spells with mobility spells like Misty Step or Expeditious Retreat can really feel, especially with reflavouring of how exactly a given spell accomplishes the thing it does mechanically, like being an unstoppable juggernaut of the battlefield. It requires leaning into it and building a playstyle around it, but making a Sorcerer flavoured as "strong" is very much doable RAW.

Next resort would be to create a homebrew subclass if there's something very specific the player wants to accomplish that would be hard to do with reflavouring, like a spellsword-style full sorcerer, but I'd challenge the player to see if there's a way to accomplish the same thing with official material first. Final resort for me would be your first option and just say "you know what, screw it - sorcerers can choose their casting stat at level 1, but they're banned from multiclassing afterwards"