My kid went to Europe every summer but during and after college.
For high school not sure if Fontainebleau FONTAINEBLEAU SCHOOLS OF MUSIC AND FINE ARTS/ LES ÉCOLES D’ART AMÉRICAINES DE FONTAINEBLEAU – At the nexus of French and American music and art since 1921. or European American Music Alliance would be available. EAMA - Discover your musical lineage. (europeanamericanmusicalalliance.org), Heavy on theory, both in person and online.
soundSCAPE and COMPOSIT are in Italy, Kalv in Sweden, etc, but most are post-high school. Maybe you could contact conservatories in whatever country he would like and see if they have a summer high school program.