I am extremely concerned based on what we all know of past cases where a young man has gone missing after separating from his friends during a night out at the bar. The nearby river someone mentioned above is also very concerning.
With John being such a big, strong young man it would be much easier for a predator to physically abduct a smaller person. Although, a knife held at one's back could probably gain their compliance pretty easily. Another way he could have been lured away from the bar and then later attacked or taken advantage of is if a male predator or predatory group used a pretty young female to lure him away from the bar with the promise of going home with her for the night. I unfortunately cannot remember specific names, but there is at least one date-rape drug that does not actually physically incapacitate you, but it makes you extremely compliant. If John was under the influence of something like that, or was being forced by knifepoint (which is probably not terribly difficult to go do unnoticed in public) then I could understand how all of his personal belongings were left behind.
When they say that John is in Rome for the semester.. sounds to me like maybe he has only been there about a month (if the semesters are similar to American Universities' semesters)? I wonder if it is possible he got lost while trying to walk home and ended up in a dangerous situation quite far from the bar?
So many questions.