Talos will be used as a brace dog - he'll help his person keep his (or her) balance. A brace dog needs to be (1) tall, (2) sturdy, (3) strong, and (4) calm. Not many dogs fit that bill... except a Great Dane.
From the breeder at Service Dog Project: The Balance support dog should be at least 40% of the person’s height. A 6-foot tall man needs a 30” dog. This puts stability at the person’s fingertips. The dog should also weigh at least half the person’s weight. Great Danes are well suited to home and office life. The fact that Danes require less exercise than many breeds is a good fit for some people with disabilities. Danes are content to have a few walks every day and then nap in the corner until their services are needed. They would not dream of chasing a tennis ball. If someone intended to climb mountains with their service dog, a Dane would not be for them.
Pictured above is a Great Dane named Arrow, who is in service as a balance dog. You can see how tall he is - his person (in the pink shirt) holds onto Arrow's orange harness for balance support. She's 5' 10" and you can see that her arm is bent when she's touching Arrow. (Arrow, by the way, is Talos' half brother - they share the same mother.)
Many Danes, if bred correctly, can reach 10 - 12 years of age. Breeding for function, not for looks, helps develop a hardy dog. Even though he might not meet the Great Dane breed standard, he's more likely to live a long life, assisting his charge for many years.