He was pissed at Johnny for a long while because Johnny was a playboy and divorced his wife (Ginny, I think?) to marry Margot Ashton, the movie star/slut. It was only when Johnny asked him for a favor at Connie's wedding that he began to forgive him, and as I recall in the novel he basically told Johnny to patch things up with his ex-wife (even if they wouldn't get back together) as a condition of his help.

I always figured he wasn't as hard on Johnny because A) Johnny was his godson, not truly blood, and B) As a movie star Johnny would be exposed to a lot of vices and, being a man, would find it very difficult to suppress those urges.