What is the ruling of Sharia concerning parents – who do not pray – preventing their son from attending Fajr prayer in congregation, for fear of neighbors' talk and harassment, and on the pretext of the cold weather? And how should the son deal with this harassment?

Attending congregational prayers in the mosque is a confirmed Sunnah. Praying in congregation at home fulfills the obligation (fard) but misses the Sunnah. If there is no way to pray in congregation except in the mosque, then it is obligatory to go to it. There is no obedience to anyone who prevents you from the obligatory congregational prayer unless you are forced to pray alone. The cold weather can be mitigated with heavy clothing and by heating water.