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I'd rule it as being grappled, which means a concentration check.
Grappling or Pinned
Casting a spell while you have the grappled or pinned condition is difficult and requires a concentration check (DC 10 + the grappler's CMB + the level of the spell you're casting). Pinned creatures can only cast spells that do not have somatic components.
Actually, you can't if the spell in question requires a somatic component.
To cast a spell with a somatic (S) component, you must gesture freely with at least one hand. You can't cast a spell of this type while bound, grappling, or with both your hands full or occupied. (CRB p.184)
If the spell uses somatic components, he wouldn't be able to cast it.
If he's using a spell with no somatic components (such as Dimension Door) then he should be able to cast it normally.
I wrote a third section about how Material Components would interact with the above before I realized the character in question is a sorcerer. xD