According to dictionary.com, "salad days" means "a time of youth, innocence, and inexperience". While this definition may not hold true for Sparkey in his golden years, his predilection for fresh blades of grass as a daily dietary staple is something to behold.
Most dogs seem to only eat grass when they feel a need to vomit, a practice which I have previously deemed "Canine Ipecac", and a habit that Tina demonstrates periodically. For Sparkey, however, this herbivorous grazing has intensified since the onset of his illness, and I theorize that he intuitively knows that the grass is a tonic for his kidneys and he sets out to eat his daily fill as a way to self-medicate using what's available to him in the natural world.
I would not characterize Sparkey as youthful in the literal sense, nor would I deem him inexperienced, but he is, by nature, an innocent in this world. So, even if "salad days" is not a perfect fit, I see this time during which Sparkey begins to transition into the next world as a period of innocence, letting go of youth, releasing the past, and opening towards a new beginning, a new "youth" of the canine soul.
Bearing that in mind, if the boy wants grass, he gets it, and may the Goddess bless his pointy little head.