Wilhelm sat, mountainous. He was not really so slovenly as his father found him to be. In some aspects he even had a certain delicacy.
This is a description of the narrator's in Chapter III, after Mr. Perls has left the breakfast table at which Dr. Adler and Tommy were sitting. It points to various elements: the point of view of the novel and the duplicitous nature of Tommy.
First of all it is important that the narrator is, at this point, taking a rare distance from Tommy's own perspective to describe the man that the narrator's voice so often inhabits. However, it seems that even in this, the narrator takes Tommy's perspective in illustrating that Tommy is not truly as his father sees him. It is important to bring this narration device into question constantly.
The language and word choice of the quote is important. First of all, Tommy is described as "mountainous," which refers to many of his qualities. He has eaten a great deal and so it is a description of a full and heavy Tommy. At the same time, however, the heaviness of the description may refer to the burdens Tommy has to withstand. It may also, however, point to a hidden strength—a strength that lies beneath his "delicacy." It is this combination of strength and delicacy, a quality his father finds negatively effeminate in Tommy, which will allow him to cry with full force at the end of the novel.