Diapers are not harmful for boys to wear.
That goes for both cloth diapers as well as disposable diapers.
Both disposable diapers and cloth diapers are perfectly fine for boys to wear and they do need to wear diapers when they are babies and toddlers and not potty trained.
The main issue with diapers including disposable diapers when it comes to being harmful to boys is that the diapers are supposed to keep the boys genitals warm and that is supposed to make the boy possibly infertile later on in life.
That would mean that the boy would not be able to get a woman pregnant later on in life.
But if diapers were harmful to boys and caused infertility in boys then nobody would be able to have any babies anymore.
Billions of babies and toddlers including boys wear disposable diapers as well as cloth diapers and they turn out just fine.
My brother wore disposable diapers as a baby and wore diapers until he was near 4 years old and he has 5 kids now of his own.
So while some men to become infertile later on in life it's not because they wore diapers.