The eligibilty criteria for bitsat is that the aggregate in PCM/PCB must be 75%. According to this criteria of eligibility your son doesn't qualify for bitsat. Here i am sharing a link with you of the official site of BitSat 2019. This will clear all your doubts.https://www.google.com/aclk?sa=L&ai=DChcSEwiDzfT55Y3hAhUHJSsKHaxEAvQYABAAGgJzZg&ae=1&sig=AOD64_1UReTd0KXSWfa-geqs4rXaJZcE1A&q&adurl=http://bitsat.in/exam-2019/?utm_source%3DMobile&ved=2ahUKEwipu-755Y3hAhWLP48KHbJSABgQ0Qx6BAgREAE