His "alone time" could be when he is sleeping, therefore he will not be able to get to any tissues.

Obvious solution: Teach the horny, little master baiter to do his own laundry! If he's stupid and/or shameless enough to let you find his crispy PJs, he most likely won't dispose of the wad-filled wads of Kleenex properly.

Oh god that's awkward. I'm so glad I have 2 girls and not boys lol

I think it's probably a safe bet that it's from wet dreams, rather than wanking off.

You might want to invest in some door locks in the rooms where he's likely to "take care of business," and advise his of their proper use. You want him to keep "alone time" all to himself. This is one case where sharing is not caring ;)

yeah I'm pretty sure that he was definitely having wet dreams

Yes, it's very important that PJs are smooth and not crispy when they go in the laundry. Sheesh...

Have him test the statistics on condoms,

Every mom does. Consider yourself lucky my sister felt 'that' while her kid was 9

if he is 13 he should already be doing his own laundry and also eeww