DS (20) will be graduating next year and wants to spend his summer travelling before he starts working. So far he's planning to go to Vietnam, Thailand, Sri Lanka. But today he mentioned that he would also like to climb a mountain. He's done small mountains (e.g. Ben Nevis) with us as a child, and but he wants to climb something much bigger and more challenging. He's very fit (sporty) but not a meticulous planner, tends to leave things to the last minute and do them spontaneously. So, I've suggested that if he was to do something like this it would need to be as part of a group, with a guide, and a kit list, etc. However, he's a student, so cost is a factor.
Any suggestions of suitably challenging but relatively safe mountains would be appreciated. (If there are no suitable options in the countries he is already planning to visit then he would consider a separate trip somewhere closer to home).