I'm still learning DW and just getting my first campaign off the ground, so I may be waaay off. In addition to a -2 modifier, some other ideas are:
Option 1:
If the fiction says he can counter spells, then he counters spells. Purple worms swallow you whole, basilisks turn you to stone, and ropers catch you unawares. It's just what they do. Counterspelling sorcerers counter spells. The players should preferably have some narrative warning, and the fiction should give them some way to interact with it (disarm the staff, attack the sorcerer so he can't dispel and defend at the same time, multiple spellcasters overwhelm him, etc.).
Option 2:
Use the existing spellcasting rules, don't change the roll, but increase the consequences. Maybe the sorcerer can redirect the spell onto you or your allies, or rechannel the magic to fuel his own spells.
Option 3:
Create a new move called "Magical Duel". When the Wizard attempts to cast a spell, staves and wands shoot different colored flames and negate each other, etc.
Roll +Int
10+: You overpower the sorcerer and may attempt to cast a spell, taking +1 forward to the roll.
7-9: Your battle of wills is grueling. You are successful and may attempt to cast a spell, but it takes a toll. Choose 1:
- You take -1 forward to the roll.
- You take 1d6 damage.
On a 6-: heheheh... Basically the sorcerer gets to make an attack, or losing the duel inflicts a debility on the Wizard, or any other suitable follow-up move.
(Or some such. I'm sure that needs work, but you get the idea.)
My own preference is Option 1.