Q: As a man, how much of my body must I cover in prayer?
A: From the conditions of prayer is covering the ‘awrah.
Grown men and boys ages 10 and older must cover what is between, and not including, the navel and the knees. A child from 7-10 years must cover their private parts. This must be covered at all times except for medical purposes, to use the restroom, or during intimacy, etc.
It is sufficient for a man to simply cover his awrah in order to pray a voluntary prayer. For an obligatory prayer it is sufficient to cover the awrah along with one shoulder. It is however, preferred for men to pray in two garments such as a long shirt and pants.
Source: Hidayat al-Raghib