First: he's become the team powerhouse. Exploit this.
Consider the Worf Effect. Have a bad guy that has bigger fish to fry (at least for the time being), and thus tosses Sir Stormforge aside like a ragdoll before moving on to their current goal. This will instantly communicate to the players exactly how horrifically terrifying this enemy must be, and thus they'll have to find a tricky solution to deal with it. The player has already done all the work of establishing how powerful their character is, so judiciously undercutting that now and then will be highly effective at communicating how scary an enemy is.
Also, consider splitting the party. It's awesome that Sir Stormforge can kick all the ass and take all the names. He can't be in two places at once. Challenge the players with delays, diversions, split goals that have to be completed at the same time, etc. This both creates alternate pressures (any group that has Sir Stormforge is secure against violent danger, but because the party is split, they may not be secure against other dangers--perhaps Intelligence or Dexterity things, and sending both the Powerhouse and the Smart Guy off together almost certainly leaves huge weaknesses for the remainder.) This enables both more interesting risks, and the opportunity for a Big Damn Hero moment where Sir Stormforge sails in to hold off the bad guys.
Second, offer temptations and manipulations. Sir Stormforge is so powerful--why does he hang out with these chumps? Or perhaps Sir Stormforge is utterly dedicated to protecting/caring for his allies. That's a huge exploitable weakness. Does Sir Stormforge have any dark secrets? Guilt trips, berserk buttons, old flames, fragile loved ones, etc. Or perhaps he has hidden or not-so-hidden temptations. The player likes to pursue power--could they handle the temptation of a dangerous but objectively very powerful addictive supplement, like a drug or a magical effect? Conversely, if they're truly leaning into the Incorruptible Pure Pureness, how can that be used to manipulate them, particularly with their comparatively weaker allies? What sacrifices could he be persuaded to make? What incredibly dumb choices could he be forced into because "honor demands it" or the like? Security is only as strong as the weakest link--if you can't pick the lock, pick the person who holds the key.
Third, offer challenges that subvert the character's power. If there are long-running bad guys, surely they'll figure out that the powerhouse is almost invulnerable physically. Attack the mind, whether directly (mind control) or through things like illusions or disrupted perceptions. Or perhaps a "mirror-match," where the players must face off against something that matches (whether loosely or precisely) their own strengths, such that Sir Stormforge's invulnerability becomes itself a liability for the party. Or something that gets stronger based on how strong the opposition is--something that can only be defeated by a peaceful, passive response, not a violent one, meaning Sir Stormforge's invulnerability and (plausible) unwillingness to back down is exactly what would make the enemy unbeatable. Or throw out things that defy melee-range actions, e.g. some kind of undead or the like which consumes the life-force of things nearby (inflicting Exhaustion on opponents in melee when striking or when struck? I dunno, there are options.) Or perhaps the party needs to be sneaky--none of Sir Stormforge's abilities are useful for concealment, and indeed pretty much all of them are really loud and bright (hammer and anvil, thunder, and divine smite; forge-sparks, lightning, and...also divine smite!)--turn those strengths into disadvantages so he has to rely on his sneakier teammates. He's probably also relatively weak on knowledge skills and skills generally, so adventures that require a lot of skill rolls or that integrate skill usage into combat is a great way to reinforce that, for all his power, he cannot succeed alone.
Don't use any given response repeatedly, or even every other adventure--variety is the spice of life. But if the player has become a one-trick pony, there are still tons of things you can do to plausibly threaten the party or create tension.
