Poor boy. I understand how he feels - or rather my DD did. We had a similar situation when she went on her residential trip in Year 6. It was a week, but only in this country.
She is a very happy confident girl, but has always wanted to be home at night. She wanted to go on the school trip, but just like your son, as it got closer, we had tears, she didn't want to go etc. I wasn't going to force her to go, but I did say to her that she might be a little homesick at times, but I didn't think she would be unhappy all week, so if she could cope with feeling sad now and again she would be ok. She did go, it was her choice, and she had a wonderful time.
All the children from the year above told them not to worry about being homesick, and said the same things that I told her. They weren't allowed to take phones, but the teachers texted us just to say they had arrived safely and everything was fine.
They were homesick at times, all of them. DD was ok mostly. She missed us at nighttime, but I told her to take a good book, and not think about us too much, and the week would fly by. I didn't want her crying in bed and making herself feel worse. She now says it was the best week ever, and still talks about it over a year later.
I do think it's very young for your son to have a school trip abroad! Has he skied before? Is that what's worrying him?
If I were you, I wouldn't force him to go, but I would persuade him that it would be worth going. The good points, (skiing, holiday with friends, all the fun they will have, and the memories) must outweigh the bad points - missing his family. Explain to him that he might be homesick at times, but it's only for a short time. Point out to him how quickly a week goes by.
I hope he goes and has a wonderful time!