Use the stunt mechanics (found on page 42) to great effect.
First, ask the player what his intent is - what does he want to have happen? Then make him use an action (standard, move, minor) and a skill check or an attack to have it happen. The key is to make the Wizard use up his actions - these are his most valuable resources.
Some guidelines:
Any time that he wants to do something that "defeats" a creature - making it surrender, run away, or fall under his control - he deals damage. Dealing damage is always a standard action, and that's all he can do with the check (Arcana, using the table of DCs + 5, or Int if it's directly opposed by a creature, like an attack).
If he wants to "defeat" a creature who is bloodied, he can do it at your discretion, but use the Intimidate guidelines - +5 or +10 to the DC, and only 1/encounter.
If he wants to impose some kind of effect, make it 1/encounter. He does not deal damage with this, and it takes a standard action.
If he wants to control other creatures on his side in combat, make him use his own actions to do so. A summoned creature might start off right next to him, and then it needs to use the Wizard's move actions to move, the Wizard's standard actions to attack (resolved using the Wizard's Int vs. AC), etc.
I'm not sure how many hp a summoned creature should have, if any. I think that you could say that any attack hits the summoned creature automatically and kills it - or, and I think I like this better, use the Wizard's defenses for the summoned creature, and if it takes damage, the Wizard takes the same amount. The Wizard determines if the summoned creature dies after it takes damage.
Illusions that make a target think something - something that does not "defeat" the creature (like, "Oh my god we must run!") - you should be able to use the above guidelines for most things. Stuff that doesn't fit - like making an illusory wall - I think you'd have to handle on a case-by-case basis.
Make an illusory wall - intent being that the NPCs can't cross the wall: Roll Arcana vs. DC + 5 (easy, hard, or moderate depends on if the wall appears in their faces or if you create it in an area they haven't been yet, and if it makes sense for a wall to be there). Failure means they see it for what it is, and can cross through the wall. Success means they can't move through the wall. 1/encounter. Standard action to do it, sustain minor.
Make it sound like reinforcements are coming - intent being to make the NPCs run: Roll Int vs. Will, 1 target, 1/encounter vs that target, and deal damage to him. Maybe a feat could increase the effect to a burst 1 or something.
Charm a creature so it attacks an nearby ally: standard action, make Int vs. AC against the creature you want him to attack, deal low damage to it if you hit.
Just some thoughts.