how to clean fingernails if u have the kind of fingernails my bf has?!

he bites his nails a lot so his fingernails are already inside his finger by a bit. and somehow dirt can still get stuck inside his fingernails even tho his fingernails is arleady inside his finger (how does that happen?) he is an art study so its currently a lil hard to tell him to lay off on dirt, but how would u clean his fingernails if i wanted to?

i clean my fingernails (underneaht them ) just by clipping off my fingernails. i would twist a tissue and clean under his fingernails but the way his fingernails are, it looks like this would be painful for him to endure. wat do u guys think? wat should i do? give the tissue trick a shock?

im not being paranoid they just reli look dirty and i could use cleaner fingernails on him for playing around

thank u