Reference Material:

https://docs.aws.amazon.com/AmazonRDS/latest/AuroraUserGuide/AuroraPostgreSQL.BestPractices.FastFailover.html
https://docs.aws.amazon.com/AmazonRDS/latest/AuroraUserGuide/AuroraPostgreSQL.BestPractices.html#AuroraPostgreSQL.BestPractices.FastFailover.TCPKeepalives
You need to set the following TCP keepalive parameters:
tcp_keepalive_time controls the time, in seconds, after which a keepalive packet is sent when no data has been sent by the socket (ACKs are not considered data). We recommend the following setting:
tcp_keepalive_time = 1
tcp_keepalive_intvl controls the time, in seconds, between sending subsequent keepalive packets after the initial packet is sent (set using the tcp_keepalive_time parameter). We recommend the following setting:
tcp_keepalive_intvl = 1
tcp_keepalive_probes is the number of unacknowledged keepalive probes that occur before the application is notified. We recommend the following setting:
tcp_keepalive_probes = 5
These settings should notify the application within five seconds when the database stops responding.