From LSJ

ὅταν δὲ τἄμ' ἀθυμήσαντ' ἴδῃς, σύ μου τὸ δεινὸν καὶ διαφθαρὲν φρενῶν ἴσχναινε παραμυθοῦ θ' → whenever you see me despondent over my situation, do what you can to lessen and relieve what is wild and senseless in my thinking | whenever you see me despondent, you must cure the grim derangement of my mind and encourage me

Source

French (Bailly abrégé)

1 dat. sg. masc. et neutre de ὅς, ἥ, ὅ;
2 poét. p. ἑῷ dat. sg. du pron. possess. ἑός.

Russian (Dvoretsky)

ᾧ: dat. sing. m и n к ὅς.