So recently I've noticed that one of the characters in my group's party has become surprisingly tough, and while that's all well and fine, I fear it may upset the balance in the party too much.

The character in question is a Lizardfolk Sorcerer/Fighter/Dragon Deciple, and for a while he was in a power lull as none of his abilities matched those who had not multiclassed, he has now ended up on the other side with alot of power.

He is only level 11, but has 95hp, 25 Strength, Atk 10/5 and 10 with a Holy Gnome Hooked Hammer as well as a bite attack, AC 26, and F/R/W of 11/4/8.
Part of this power is from his more recent purchases, which include the party putting enough money together to get him the Belt of Giant Strength +6, something I didn't think they'd go for for a couple more levels at least. His Gnome Hooked Hammer has the hammer end as Holy, which he will Power Attack with two-handed for huge damage. And his armour is Mithril Breastplate with the Twilight enchantment for reduced Arcane Failure.

He's managed this becuase the party has been kind enough to lend him money for the more expensive items knowing that he'll be able to tank better, but now I'm faced with the problem of enemies finding it difficult to scratch him while the others (being a rogue/shadowdancer and a ranged-weapon cleric) take only a few hits to fell. They of course have their own magic items to help themselves but not as powerful as the Lizard's, so they themselves struggle sometimes to hit, let alone do as much damage as the Lizard does.

The one other thing that he does is in his buff. He casts Enlarge Self and follows that up with Rage, to give him a +4 to his strength and -3 to AC. If he does that and power attacks (likely with True Strike active instead of Rage) two-handed on an evil enemy, he's going to crush them, too quickly.

Aside from an antimagic field on him, what could I do to make it fairer? I may take away one or more of the magic items, but I won't be doing it too soon considering he only just bought the belt and has had the hammer as Holy for a session or two longer, they'd feel cheated and like it was a waste of money to do it too soon.

What to do? I'm looking for ways to make him a little more vunerable where the other party members could help. Any ideas?