I think since you have his family's blessings it is okay to go ahead.
It is apparent that your heart is in the right place, and I believe you will come up with something righteous.
As a former surfer, I am sure that it will inspire him to get through therapy, learn to use that prosthetic leg, and learn to surf again.
Be sure to include some pix/stories of others who have made the journey back:
Also include the inspirational story of Natalie du Toit of South Africa, who qualified for the Beijing Olympics despite losing her leg in a motorcycle accident:
Here is Natalie's story
Tell your friend that people who don't even know him, wish him well, and that they hope he will be back on a surfboard communing with the waves again soon...
ADG