wonnie
2 options
Your sons friends parents can add him to their Insurance policy for an additional premium.
Or you can phone Insurers to arrange a single trip policy for your son. The policy will be in your name, but will note that your son is the sole traveller.
Personally I would recommend the first option as if anything happens to your son while away, your sons friends parents can sort out, through the one Insurance policy.
Huckster