If it's a tension release for him, have a tactful, loving conversation about him trying other things to relax -- like going to the gym or participating in sports.
If he masturbates for sexual pleasure, then you need to have honest, tactful, unheated conversations about how this makes you feel. Try to get to the bottom of why he masturbates frequently, and then you can start to move forward. It's important for him to gain more empathy about your feelings. You can try the approach of asking him how he'd feel if you masturbated daily and then ignored him in bed. The goal is to get him to put himself in your shoes.
I always say that masturbation isn't a problem unless it's upsetting a relationship -- as it is in your case.
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Dr. Pam is the author of Make Love All Night & Talk to Him in the Morning and Naughty Tricks and Sexy Tips: A Couple's Guide to Uninhibited Sexual Pleasure