Poor Jim is just your average New England dude who wants to enjoy life like Strether and Chad. Think that charmer from the office, but instead of getting his very own Pam he has the misfortune of being married to Sarah Pocock, who like her mother has learned very effectively how to make men do what she wants. She's basically the anti-Pam.
This means that Jim is on cloud nine anytime he's able to get away from Sarah, yet he's also scared enough of her to know that he can't openly disobey her. That's why he's so pumped when they arrive in Paris and Sarah wants some alone time with Chad, as the narrator notes, "He gurgled his joy as they rolled through the happy streets; he declared that his trip was a regular windfall, and that he wasn't there, he was eager to remark, to hang back from anything" (8.2.12). Gurgled?? That must be one heck of a bad marriage.
The trip to Paris gives Jim the same chance that Strether has had to experience a little bit of independence before he gets dragged back to his boring, emasculated life in Woollett, Massachusetts. And when it comes to identifyfing what's wrong with Jim's life, Strether puts his finger on it: "What none the less came hom to him, however, at this hour, was that the society over there, that of which Sarah and Mamie—and, in a more eminent way, Mrs. Newsome herself—were specimens, was essentially a society of women, and that poor Jim wasn't in it" (8.2.12).
Yes, Strether could just as easily be talking about his own life here. But the difference is that Jim is resigned to his boring life in a calm, cheerful way that Strether just can't stomach.