Erised

The balance required to cast the Patronus Spell was not easy to achieve. The strength of the spell hinged on several factors: the wizard's attitude, his experiences, his traumas. Whilst a kid might find it relatively easy to focus on a happy memory, for a grown-up, life experiences could easily alter the already complex fabric of the Spell, and ultimately alter happiness itself, so the Patronus was likely to turn out weak, or delayed, or it might dissolve in an instant.
One could completely lose the ability to summon it.
But her voice, her fire maybe can give him a dream.
Only she could give him a thought happy enough to recall his own from past memories again.
Just a thought, because a memory was beyond his reach.