docmidnight:

First off Grandpa Harley wears pretty much the same outfit as TR’s iconic safari getup: 

Theodore Roosevelt was a notorious hunter and learned taxidermy at age 6 and after a hunting trip to Africa he started the Smithsonian Museum to showcase all of the exotic animals he had stuffed.

Theodore Roosevelt as a child:

Roosevelt was a sickly asthmatic boy, who ended up learning boxing and wrestling to improve himself and his body.

Roosevelt was also keenly interested in cataloguing flora and fauna (and would have been comfortable using the words in conversation) and he wrote a dissertation on the distinctions of different types of animals at age 9.

A young Roosevelt was also a big fan of pulp novels, which were predecessors to comic books.

Also some of the qualities Roosevelt saw in his father that he hoped to exemplify in his own life were “strength and courage with gentleness, tenderness, and great unselfishness.” Which seem to be qualities that Jake would like to posses as well.

Finally, Jake’s old fashioned manner of speaking matches up with the way TR would have talked, as does both of their fascination with technology.