I think my reply to that would be that he hasn't offended me, but as someone who has admitted to occasionally still smoking and has admitted to not using the right sun factor for even British summertime, yet still promotes your charity is very ill informed and not someone who you might want promoting Sarcoma and cancer charities.
I'd also call their bluff and ask that in his role as an ambassador, is it not standard that he sits in on training and meetings where the dangers on cancer and preventative measures that are discussed and therefore should know the full, or added risks - especially when he frequently fully admits his health anxieties thanks to his brother and dad's untimely deaths due to cancer?
You could go further and say that as a parent he doesn't show his children in UV suits and hats whilst in hot climates and that they have notable sun tans, which any cancer research services will advise against.
I'd also call them out that as a charity that you can understand their need for 'high profile' professionals speak for them and raise awareness and understand that they need any help they can get, but that they might decide to use someone else that actually promotes safe sun for adults and small children. What he is doing isn't raising awareness, it's promoting unsafe practice and increasing the risk of cancer
And yes, I realise he's not high profile, but as a charity they will see him as that believe me, so swallow your pride, suck it up and get the sick bucket ready when you reply