cloa513ch2629-san,
that's an interesting question!
I think there's no perfect translation in Japanese.
Traditionally, being dependant on mother, being soft and so on were considered as "sissy" in Japan
and not at all positive. We have the word マザコン which means "mother complex" and this word used to be
used as criticising a man who cannot be separated from his mother and/or be a man.
Recently, it's getting more and more "acceptable" for men to show their soft and caring side including
love to mothers. So, this マザコン has just started to be used also as "good", but still, not 100% positive.
The expression "mummy's boy" seems to have more meanings and with positive concept, so
we might use several adjectives to describe such persons.
If you meant negative only, maybe マザコン would be the best fit...
Natsuko(奈津子),
Team JapanesePod101.com