J.B. Handley is the founder of Generation Rescue (GR), an organization advocating the view that autism (and related disorders) are primarily caused by environmental factors, and in particular by vaccines (shifting goalposts for the previous “autism is misdiagnosis of mercury poisoning”). For anyone in doubt, these claims are biologically implausible and lack scientific evidence. But evidence hardly matters. GR may be the most famous of the anti-vaxx groups, and certainly among the most aggressive, gaining widespread attention from aggressive media campaigns (including sponsoring full page ads in the New York Times and USA Today). Today, Generation Rescue is known as a platform for Jenny McCarthy's autism advocacy, but Handley is still pulling most of the strings. The wikipedia article is actually fairly balanced. He is generally known for supporting discredited researcher Andrew Wakefield and offering arguments of dubious coherence.
He also founded and has an editorial role (but denies that) in the Age of Autism community (AoA), which basically does the same as GR (it’s a blog/news community) at a broader scale, but with the same level of reason and accountability.
The AoA has repeatedly called for a civil debate on vaccination. Some of Handley’s contributions on behalf of AoA to the civil, reasonable debate are covered here (a good illustration of how denialist movements actually work). See also this, or for the truly bizarre, this.
He has absolutely no understanding of science either, and has confirmed that if scientists say something that contradicts his belief that vaccines cause autism, then they must be corrupt and lying.
Diagnosis: Dangerous, zealous crackpot – more precisely a hysterical pinhead unable to distinguish scientific evidence from emotional reactions. Unfortunately he is rather influential and a real danger to civilization.