Stress at the level he appears to be under is probably not a healthy stress, and won't help him to perform better (as "good" stress usually does). It sounds like he's feeling overwhelmed at present, and supporting him in sorting it out is the best you can do. Remember tho, that this is his problem primarily and he needs to resolve it so listening to him and asking appropriate questions will likely be the most helpful. It might also help to assure him that feeling stressed when doing a new job is entirely normal - everyone will feel it to some extent.
It will likely help for him to define the problem. Sitting down and writing out (as a list) what exactly are the problems that are causing anxiety. Be specific, what's worrying him exactly, and then prioritising the most important issues that needs to be addressed. Doing up a work plan to address these should help to clarify and deal with the problems. Speaking to his boss should also help, particularly clarifying what is expected of him. Maybe getting some assistance in certain areas if needed - ie you mention this is Europe-wide and he only speaks English, does he need help here? He may also need add'l training to help him cope with a managerial/supervisory role. Maybe a mentor would help?
Often it is the case that good sales people are promoted into managerial type roles without adequate training in the new role, and they struggle to perform. Again, this is normal, and can be greatly helped with training & adequate support. I don't want to generalise, but it is often the case that career-focussed men don't ask for support and internalise the anxiety. The sooner he deals with this problem, the sooner he will begin to feel a level of control over the matter, and his sense of being overwhelmed will reduce. It might also help to have some ways to let off steam, like vigorous exercise, movies etc. This will mentally give him space to see the problem more clearly and deal with the physical affects of stress. I wish you and him well with this.