To stop with the sexual innuendos as they're making me uncomfortable. I don't know how is best to handle this situation, small office, no hr based on site and line manager is a bit wank in all honesty and never around to hear anything.
He is 20 and has an awful professional manner, which I think is why I guess I'm holding back because a part of me keeps telling myself it's his first proper job, he's young don't be too harsh etc.
But the other part of me thinks fuck him, he shouldn't be in a professional environment if he can't behave accordingly. Few examples...
Mentions his "large" penis several times in a serious way 
Shut the door on the small storage room I was in and blocked my exit saying "you're mine hahaha". I actually panicked and felt fucking terrified for a split second.
I was mumbling to myself about something that had gone wrong and when he asked me I said "oh nothing I'm just moaning"....his response was oh you're moaning, well don't let me stop you followed with actual sex noises and well I don't mind hearing that.
It's making me so fucking uncomfortable, I have to refrain from throwing the printer in his fucking face every time I see the creep.
It's hard to explain but the lingering smirks and looks are just fucking odd. He stands in my personal space at every opportunity.