I think it's a very natural thing, especially with that kind of profession. If he chatted to his classmates then I'm sure that he'd find he is not the only one.
Even on placement, he could have a chat with his supervising paramedics about how they deal with things like that. They've been through it and know what it's like, might have some tips, he is there to learn about the profession, not just the practical side medically. I'm sure they all have their own stories to tell, they were all younger and students at one point.
He's good with people and with his training and experience he will build up confidence. A good tip someone gave me once was that even if you don't feel it, try acting it (as in actor kind of act it). Sometimes just pushing that boundary a little in that way feels a bit less of a leap and less threatening, but then it can help you realise it's not that difficult doing it, if that makes sense?
The more he goes out, the more different situations he is in, the less impact a memory like that will probably have. He will be dealing with people in extreme circumstances a lot of the time, a lot of whom won't be at their best, I'm trying to think of a better phrase but I'm too tired, which is no reflection on him.
In a way I think comments like that are more about the other person. When you're older and people in certain professions start to look really young, with me it's more about me feeling older than anything to do with them :)