I've said this on the forum a few times now, but he honestly just needs to suck it up (man to man here/tough love). I drank a minimum of 60 oz of soda per day. It went down something like this: 44oz soda from the corner store in the morning to get me to lunch. Water or a 20oz soda at lunch. One or two 12 oz sodas at home in the evening.
I took to Spindrift my first Whole30 and also found a compliant Spearmint Hot Tea to give me a bit of caffeine help. I had horrible headaches for a few days but then the SpinDrift and Water alone were fine. By the end of my first Whole30 I could no longer stand the tasted of diet soda. Now when I absolutely feel like I want one I'll oblige myself with one regular sugar soda but otherwise it's water, sparkling water or SpinDrift. During my current Whole30 I have had tea once or twice a week and drink at least 80 oz of regular water and (2) 12oz Sparkling Waters of various types a day.
Yes it sucks, but it will likely change his perspective on those drinks. This is still one of my biggest NSVs of Whole30 and it has lasted over 6 months!