Is it forbidden for a man to uncover his upper half or to wear a swimsuit or shorts in front of someone—even if in his own home? And is a man's nakedness (awrah) only the private parts, or is it from his navel to his knees?

It is permissible for a man to uncover what is above his navel and below his knees, because his 'awrah (private parts) is between the navel and the knee, according to the most preponderant opinion among scholars. The evidence for this is the Prophet's (peace be upon him) saying: "What is between the navel and the knee is 'awrah," and his saying: "Do not uncover your thigh, and do not look at the thigh of a living or dead person." Nevertheless, a man's uncovering what is customarily covered may be considered a breach of muru'ah (chivalry/manliness) that compromises one's 'adalah (uprightness/integrity), even if it is not a sin. This varies depending on individuals and countries, so this should be taken into consideration.