"Rising" sophomore (or junior or senior) is a common U.S. expression. It just means he'll be a sophomore when school resumes this fall. There's no self-promotion, it is just used by students during the summer break.
It's also worth pointing out that I'm US born and bred, spent 7 years at universities and was unfamiliar with the term. It sounded pretentious to me.
It's a useful expression when you deal with college students over the summer.