Some 8 year old boys do wear diapers.
It's okay if the 8 year old needs diapers and wears diapers as there's no age limit for wearing diapers.
Some 8 year old boys may have bladder incontinence, bowel incontinence, weak bladder, ADHD which causes daytime wetting and even bed wetting and some kids just wet the bed.
Even if the 8 year old wants to wear the diaper and they are happy doing so they should.
Diapers don't have an age limit although most people associate diapers with babies and toddlers.
Anyone of any age can wear diapers and there's nothing wrong with it.
Diapers are just another form of underwear and nobody should be ashamed of wearing diapers whether for need, comfort etc.