Impurity emerging due to a leaky gut

If the wetness is due to perspiration only (as you have stated in your query that when he sweats his clothes becomes wet), then the place and his clothing will be considered as clean. However, if the wetness is due to impurity which emerged from the rear passage and together with the perspiration caused the clothing and area of sitting to become moist, then it will be considered as Napaak and will have to be washed. A leaky gut could lead to diarrhoea which means that the impurity that emerged would not be in small quantity, rather it will be a substantial amount, thus it would cause the affected areas to become Napaak.

Notwithstanding the above, if you are referring to the minute remains of impurity after Istinja that could have transferred onto one’s clothing or place of sitting due to excessive perspiration, then this will be overlooked and not cause the place or clothing to become impure. [Re: Fataawa Qasmiyyah Vol:5 Pg:185, Aap Ke Masaail Aur Unka Hal Vol:3 Pg:176]