I think here καθ’ όσον δύναται is impersonal, am I right? οὐκοῦν ὅλως δοκεῖ σοι, ἔφη, ἡ τοῦ τοιούτου πραγματεία οὐ περὶ τὸ σῶμα εἶναι, ἀλλὰ καθ᾽ ὅσον δύναται ἀφεστάναι αὐτοῦ, πρὸς δὲ τὴν ψυχὴν τετράφθαι;
I think this could be impersonal like you said (cf. LSJ III s.v.δύναμαι) or personal with an ellipse of the pronoun. Either way it still refers to the hypothetical φιλόσοφος referred to in this discussion.
[Look back to 64d: φαίνεταί σοι φιλοσόφου ἀνδρὸς εἶναι κτλ. Picked up continually throughout. τὰς ἄλλας τὰς περὶ τῶν τὸ σῶμα… ἡγεῖσθαι ὁ τοιοῦτος 64e: ὅ γε… φιλόσοφος… ἡ τοῦ τοιούτου πραγματεία…]
And finally in the passage you’re concerned with: ἀλλὰ καθ’ ὅσον δύναται.
Plato has already used an impersonal construction after the phrase καθ’ ὅσον several lines above - καθ’ ὅσον μὴ πολλὴ ἀνάγκη (sc. ἐστιν) - and so this interpretation seems tempting. However according to the LSJ this construction is mostly negative and takes aorist infinitives, not perfects (with a positive construction later in Plutarch). As a result, my reading is: ἀλλὰ καθ’ ὅσον δύναται οὗτος, i.e. an ellipse.