Testicles need to be a few degrees cooler than the rest of the body for proper sperm production. Anything that holds them up against the body, including tight underwear, can cause them to be too warm and reduce sperm production, and maybe even delay development. However, millions of boys grow up wearing white cotton briefs and that doesn't appear to affect their development. The problem with disposable diapers isn't so much that they are too tight, but the plastic backing prevents air circulation, which can also keep the testicles too warm. If your son wore diapers 24/7 that might be a problem, but if he is only in diapers at night that is most likely not long enough to cause any problems. At age 12 he may not have begun to develop yet. Give him another year and if there is still no sign of development then have him checked by a doctor.