While Professor Proton's death was already a sad scene in The Big Bang Theory, Young Sheldon made this moment truly tragic with the spin-off's revelations about George Sr.
In The Big Bang Theory season 7, episode 22, "The Proton Transmogrification," Sheldon was initially seemingly unmoved by the death of his childhood icon Arthur Jeffries. However, an earlier episode of The Big Bang Theory already proved that Professor Proton meant a lot to Sheldon growing up (as evidenced by Sheldon's autograph collection, a running gag Young Sheldon set up years later). This led Sheldon to eventually commiserate with Leonard over the loss and admit that he was moved by Professor Proton's death, too, as Sheldon said this was as sad as losing his father and his grandfather, two other formative figures Sheldon looked up to.
Young Sheldon proves that it makes sense for Sheldon to admit that he looked up to his father in "The Proton Transmogrification." The fact that Sheldon rarely referred to his father as a role model or aspirational figure outside of this scene could be seen to negate this reading, but Sheldon's rare choice to hug Leonard further implies he is being sincere.