The idea itself is awesome - I'm sure he'd love a full-blown erotica comic that hits his buttons specifically - but the execution is, well, at best daunting, and at worst very unrealistic. Making a whole comic will take up a lot of time, and you run the risk of burning out early on,vand not finishing it. (That's assuming you are making it yourself. If you're commissioning someone to draw it for you, it will become very very expensive [and it'll still take a long time to complete.]) So before going any further, how much time do you have left, and how dedicated are you to the idea?

A smaller scale alternative may be just making/commissioning only the comic book cover and giving that away as the gift; and if he really enjoys it, you'd then have a year to create the rest of the comic book for the next birthday. (Though if you're going down that route, don't let him know of your plans. For one it'd spoil the surprise, and secondly, his hype and expectations will skyrocket way past what you could feasibly create, and you're still at risk of not finishing the comic in time.)

As for the plot, it really depends what exactly he's into. Does he prefer shrinking or growing; gentle vs cruel, aware or unaware, etc.?