My mom's boyfriend is coming on to me

Im 18, and it's been a few weeks since my mom's been dating this guy. He's like 45-48. He works in the same building where I live. Me & him get along well, I feel like he would understand anything I tell him. A few weeks back I was wearing a tank top, && I felt him looking at my boobs. Then when we were going to the beach, we were talking about cute bathing suits , and he said ' I can't wait to see how you look ' im like what the hell ?!. After I finished tanning I put my toes in the water , and he came beside me and said ' im dying to bring you in the water , im gonna grab you ' I didn't want to go in the water because the waves at Jones Beach were crazy , but I did anyway. And then he said ' I knew you would look nice and wet '

I told my bestfriend , but it's up to a point where I think if I tell him more of what's going on he'll kill him,!

So a few days ago, him and my mom were sitting on the couch, and I was infront of them giving them a glass of water , so as I poard, I saw him looking at my crotch -_- I felt really uncomfortable.
I don't want to ruien my moms happiness . But I don't want to be around him either.
Then I had an uncomfortable dream of him fingering and licking me , now I can't get it out of my head.

He had took my mom and my brother and me to my favorite resturant ' Olive Garden ' and he said that he only took me their cause it was MY fav. && he plays my fav. music in his car for me. Also Everytime were having a conversation , he try's to give me an idea of where he wants to take it Or he looks at me like he wants to eat me ! Ugh.

I need some advice !
Please help , What should I do ?