If your DS is contracted, he has many opportunities - Airborne, Air Assault, Mountain and Northern Warfare, CDQC (dive), Sapper School, CULP, CTLT, DCLT, Robin Sage, and various internships. Some are harder to get than others, and almost all are available in much fewer numbers given the current budget climate.
To be eligible to attend the summer after MSI year, you must be a contracted scholarship cadet. As an MSII, either scholarship or SMP. All MSIIIs are contracted - CTLT and/or internships are the most common opportunities combined with LDAC the summer before MS IV year due to time constraints.