Need some advice on first impression of my niece's boyfriend?

Just met this dude in the materinty ward no less. My sis tells me in code that this dude is a real dirt bag. My neice is only 17 and nine months pregers. I go to introduce myself and he gives me the matcho hand shake with the kung fu grip. I look him dead in the eyes and return his challenge with a stout grip of my own, and ask him. "What's this?" to which he says "Just a hand shake." I then tell him I'm her Uncle and love her very much so be sure and understand that her welfare is my first concern. I get the feeling he's the type that might slap her around a bit to which I feel powerless to do anything about without first being called on to act. I'm 53 now & he's maybe 20 or 21. There is no doubt in my mind I could educate this young man and I'm talking old school as is my rule. This kid thinks this old biker isn't much of a challenge but if I find out he's smacked my little girl around....well like I said I'd like to hear what y'all think. What should I do? Serious now.