Well, first, wet dreams stop as you get older. They usually stop as you move beyond your teen years, so that in itself is not alarming at all.
There are lots of things that can cause weak erections. It might be from diet, from stress (I imagine prison is stressful), or a medical issue like high blood pressure, prostate issues, or even a heart issue.
Is he experiencing any other muscle weakness?
Any heart symptoms? Dizziness? Pain anywhere? Is he able to ejaculate? I don't know if he masturbates in prison, but if he does, is ejaculation okay? What about any urination problems?
If he has any other symptoms, he should talk to the doctor there.
You also don't mention your brother's age. The older he is, the more likely it is that he'd have prostate issues.