The idea of Theo being a dog is just as intriguing to me as him being a kid.
Buck getting diagnosed with ptsd after the electrical call on top of everything else and Eddie being the only one who can really get it in that very specific way, and it's partly because he also has been there for Buck's worst moments-- but always on the other side of them.
In both cases Eddie's been the one to pull Buck out and get him back, except, much like the desert where it all began, where Eddie nearly lost his whole squad only to lose them anyway years later, he couldn't save him.
So they offer Buck a service dog and his name is Theo and he's the patron saint of soldiers and everyone thinks it's a good idea, something for Buck to take care of that would take care of him in return.
And no one has to know that maybe, for Buck, it's more that--
If Buck has this dog there with him all the time, something constant that pulls him out and gets him to safety before he starts to spiral, maybe it's almost like having Eddie there. And if he can't have Eddie there, not in the way that he truly wants, maybe this dog is the next best thing.