Felon said:Famous last words.
Sounds fairly lame, as you describe it. My first order of business would be to pull every one of those magic items off of him. They're the party's now.
Riiiight, like the utterly defenseless man is going to walk around with magic items that anyone can just take away from him.
That's exactly what they tried to do, and they learned the hard way not to do it. Life's a bitch for the PCs if their enemies have high-level wizard parents who custom-make defensive charms for them.
The balancing act with his items was that none of them were offensive. He couldn't hurt a fly (at least in part because his father knew that if he had a potent offensive item, he'd use it just to show off). But if he was placed in danger, he had enough defenses to remain reasonably safe for at least long enough for his family's knights to teleport to his rescue.
And as far as dealing with this guy, they had to roleplay it. Even if they could kill him or slap him around, they were screwed. They had at least two very public, contentious run-ins with him in the past. In his reports to his father, he had mentioned them as a threat.
What they figured out, and what they put to good use, was that the guy was a total, slimey capitalist who happened to hate an even bigger, more powerful evil guy in the city. Granted, he hated that BBEG because he wanted to displace him, but the PCs played it right to force him into an alliance. As long as they made his nasty business dealings unprofitable (selling weapons and armor to githyanki, drow, and barbarians) and gave him the carrot of helping to defeat the BBEG, he gave them some help.
One of the more interesting things about running a political campaign is that it's fun to find a place for evil people in an otherwise "good" society. The king may need an assassin at times, and if he isn't a paladin he very well might bend that LG alignment to get things done. If he does that often enough, especially if it's easier than doing the "good" thing, he might end up with some very nasty people in very permanent places in his kingdom's political geography.