I really can't imagine taking the parenting stand of making misbehavior very very easy and available, and then punishing the child. Your husband sounds a little "off" frankly.
And here's the deal. If a 9 year old boy can look at pics of naked women, he will. It's normal. That's what they want to do.
I'm a little surprised your husband doesn't know that about boys, having been one himself.
Honey, either your son can get at porn by using his dad's phone, or his dad blocks the porn from the phone, but there is no punishing a 9-year-old out of looking at prurient content if he can get to it. Lots of grown men can't say no to porn. How many 9-year-olds do you know who have this kind of self-control?
Talk again to your husband (does he refuse to block it because it would keep himself from being able to look at porn? Can't he just get a locking code for his phone?), or get someone he (your husband) respects to talk to him (your husband), about the entrapment angle he is recommending as a parenting method here. Frankly, it seems about as smart as putting a chocolate cake out on the counter every single day and punishing your son when he inevitably sneaks a lick of the frosting.
Basically, you have to decide, as a couple, whether or not you want to choke off your 9-year-old's access to porn. Letting him get repeatedly into trouble and then coming back with punishments seems like a foolish (and kind of mean) way to try to cope with this. If you can't see eye to eye on what to do, talk it over with a family counselor. I would bet they will recommend not providing the child with access, versus providing the access and then coming down like an avenging angel.