"He was in that pitcher with Mel-vine Douglas?"
Sunny is probably referring to Captains Courageous
(1937), where Freddy Bartholomew plays Melvyn Douglas
' son, who falls overboard when a prank goes wrong. Holden couldn't be as young and cute as Freddy Bartholomew, but his character may be similar to that of Harvey Cheyne, Bartholomew's character in the film (see this review
, in particular the description of Cheyne). So Salinger might have been indicating to his readers, who would have been more familiar with the film than we are, that Holden is going through a similar immersion and transition as a result of his brother Allie's death, and the grief he still carries.
The film was based on a novel by Rudyard Kipling.