Perverted ex-pol Anthony Weiner is headed to prison Monday for sexting with a 15-year-old girl — a crime he called his “rock bottom” after years of career-killing online antics.

The disgraced Democrat spent his Sunday holed up in his Union Square apartment, where a source in the building spotted Weiner’s dad, Morton, getting Starbucks around 11 a.m.

Father and son then spent about five hours together ahead of Weiner’s trip to the Federal Medical Center in Devens, Mass., where’s he’s under orders to arrive for his 21-month sentence no later than 2 p.m.

The lockup — on a former Army base about 40 miles northwest of Boston — is just one of two sites that offer residential treatment for sex offenders, who share a common housing unit and undergo therapy five days a week.

Other infamous inmates include inside trader Raj Rajaratnam, who suffers from diabetes and high blood pressure, and Peter Madoff, a reported cancer survivor who helped older brother Bernie Madoff scam $20 billion from investors through his epic Ponzi scheme.

Weiner, 53, twice broke down in tears when he was sentenced in September, following earlier scandals that forced him from Congress in 2011 and sank a comeback bid that saw him leading the 2013 mayoral race before he was exposed as X-rated cyber-sexter “Carlos Danger.”