Mine is afraid of mountain lions on hiking trails, avalanches down ski slopes, tsunamis near beaches etc. So, I have a fair bit of experience with this. What I do is to take DS a couple of hours ahead to the ski slope and just let him watch kids younger than him hit the slopes enthusiastically and eventually ask him to try it. He usually does and his fears of great big avalanches like he sees in documentaries are gone by then. Same with beaches - we walk along the beach for a while and then, I head out to the water alone and eventually he follows me and he is fine with beaches after that. We are still working on the fear of mountain lion attacks during hikes - there were a few in our area, which does not help much - but, we had a park ranger talk to DS about how most of them avoid humans etc.
So, basically, I too recommend taking it slowly. I also recommend giving your DS a choice of whether he wants to do it or not so that he does not feel pressured. I always tell DS to observe and then decide if he wants to take up the activity. I also tell him that the rest of the family will do the activity while he waits and we will arrange for one of us to stay with him if he decided against going. Mostly, he wants to participate after he sees that other people are coming to no harm and that they are having a good time.
