Why would you put your son who is weak and soft, a man who is absolutely not made for being a bodyguard or an entrepeneur into such circumstances?
Vito had no Mickey Mouse nightclubs for Fredo to run.
Seriously: Making Fredo his bodyguard was another sign that the Don was slippin'. He'd gotten so comfortable with being Numero Uno that he never imagined that he'd need a real bodyguard because he never imagined that anyone would have the nerve to attack him. Fredo as bodyguard is part and parcel of his underestimating Solozzo.
