Hey Princess,
Exactly the same thing happened to me when I was a junior. In fact I was terrified and mortified on my first Supie flight. The purser insisted on grabbing me on every occasion and practically chasing me around the cabin.
Unfortunately, many of these sleaze-bags relalize their power over new crew or "fresh-meat" as they like to refer to them. Even several years on I still come across the odd wandering hand or encroachment of personal body-space. I can normally deter this with a disgusted and scornful glare and sarcastic remark but it's taken years of experience! It's difficult as a new-joiner to not feel intimidated by a senior, but remember, "we're all part of a team" and "communication is essential". Next time you're in this situation, as you would with inappropriate behavior from a pax, explain that you have a problem with him being quite so intimate. If you don't want to rock the boat you can always ask his advice on how to support your close (fictional) friend who's taking a senior to the cleaners on grounds of sexual harrassment or merely brag about your boyfriend who is built like the proverbial brick outhouse with a balck-belt in tae kwon do and insanely jealous!
Good luck, and be strong!