What does Gatsby mean when he tells Nick, "I can't say anything in his house, old sport," in Chapter Seven of "The Great Gatsby"?

This is Gatsby's admission that he is helpless to press home his temporary emotional advantage over Tom Buchanan. Nick, Gatsby, Tom, Daisy, and others are together in Tom's huge house in brutal heat, and Daisy has just let slip, though an apparently innocent remark, that in fact she is in love with Gatsby:

“Who wants to go to town?” demanded Daisy insistently. Gatsby’s eyes floated toward her. “Ah,” she cried, “you look so cool.”

Their eyes met, and they stared together at each other, alone in space. With an effort she glanced down at the table.

“You always look so cool,” she repeated.

She had told him that she loved him, and Tom Buchanan saw. He was astounded. His mouth opened a little, and he looked at Gatsby, and then back at Daisy as if he had just recognized her....

If Gatsby were secure and hard enough to put Tom down decisively, this would be the moment. Tom is stunned, and as vulnerable as he ever will be. But Gatsby cannot do it. At the crisis, he flinches:

Gatsby started to speak, changed his mind, but not before Tom wheeled and faced him expectantly.

“Have you got your stables here?” asked Gatsby with an effort.

“About a quarter of a mile down the road.”

“Oh.”

....

Gatsby turned to me rigidly:

“I can’t say anything in his house, old sport."

Tom, back in command, changes his mind about Daisy's suggestion and insists on making the trip into town that will result in the death of Myrtle and thus in Gatsby's death.