Unearthed Arcana has an optional rule. A sorcerer or wizard can trade in a familiar for an animal follower equal to half their arcane level. This rule works great for multiclassing. Just allow the owl to continue as an animal companion adding the druid level and half sorcerer levels.
If he goes into mystic theurge I'd recommend letting him get the Leadership feat and just continue improving the animal companion (adding either all or half mystic theurge levels, you'd have to check the balance).
I have a character heading toward mystic theurge using these optional rules and so far it seems to work well. He doesn't have to juggle two weak animals and his power isn't unduly increased (gave up a familiar and a feat to get a continuing animal companion).