You could have still cast any spell
Why can you still cast while in this position?
A prone, restrained, and grappled Fighter can still attack using a 2-handed weapon, but with disadvantage. A Ranger can still attack with a longbow from this position, but with disadvantage.
Swinging a sword or firing from a bow is not simple when you are standing up -- don't underestimate that effort. Swinging a sword is not just moving a big knife around, and firing from a bow is not just pulling a string. These skills are much harder when you are also prone, restrained, and grappled. However, it is still possible to do these complex actions under these conditions.
If non-casters can perform complex movements like that, a Wizard should still be able to cast any spell (with disadvantage if it involves an attack roll). Their somatic components just have to be present, but they don't require a degree of precision.
You also could have still cast a spell with Material components, if the players hadn't specifically stolen his component pouch or arcane focus. Material components just need to be held while casting.
Things your wizard could have done
Here are things you could have done, aside from Misty Step:
Blink (3rd level spell) involves VS, and allows your character to jump to the Ethereal Plane. When reappearing in the Material Plane, you can choose any unoccupied spot within 5ft of the point you disappeared from.
Shield (1st level spell) involves VS, and allows you some reprieve since it can give you +5AC against all their attacks for 1 round. This may offset the Advantage they have on their attacks against you.
Thunderwave (1st level spell) involves VS, and pushes everyone within 15ft away from you by 10ft if they fail their save, instantly removing yourself from the grapple.