His Dad? Err, the last time Brock allowed his dad to run the gym, he nearly surrendered the Gym to Lola, who then nearly got it banned due to it being "Uncool", and Unsafe for the rock Pokemon.
Also, Forrest is most likely the Gym Leader of Pewter Gym (otherwise, why did he entrust him with Onix, Geodude, and Crobat when he could have entrusted the pokemon to his Dad?), though why didn't he try to stop his mom from allowing the Gym to be redecorated again (Especially considering the fact that these "Painters" were very shady people who were trying to steal their Pokemon). I mean, since he is the Gym Leader, he should be able to decide what the Gym needs and everything, not his Mom (Especially since the Mom nearly killed the Gym the last time she had influence, not to mention hired Thieves to completely tarnish the gym with Graffiti and took a vacation by said thieves machinations.).
Basically, even IF Brock doesn't like running the Gym, considering how the Gym was ruined Twice (and the second time around, Forrest didn't even try to stop his mom, despite being above her due to his being a Gym Leader), and how Flint chickened out, he should have realized that only he can properly run the Gym. I mean, if Misty realized this, why can't he (Especially since Lola's the Pewter version of the Sensational Sisters)?