‘That’s My Boy’ might be the funniest Adam Sandler comedy in a decade, but audiences wouldn’t know: the R-rated laugher earned just $4.6 million. That would give Sandler his worst opening weekend since ‘Reign Over Me’ in 2007, and his worst for a live-action Happy Madison-production since ‘Little Nicky’ in 2000. Things weren’t much better for Tom Cruise. ‘Rock of Ages’, which features a flamboyant and all-in performance from Cruise, grossed $5.35 million. Based on that start, the musical could total around $15 million for the three-day weekend, which would put it behind holdovers ‘Madagascar 3’ and ‘Prometheus’. Ridley Scott’s much-discussed R-rated thriller pulled down $5.8 million. That number would represent a nearly 60 percent drop from last weekend’s $51 million opening, meaning word of mouth on ‘Prometheus’ isn’t as strong as Scott and studio Twentieth Century Fox would have liked. As for ‘Madagascar 3’, the film grossed over $10 million.