Step-grandpa tried making a move on me. Help!?

My maternal grandmother remarried a year ago. I have absolutely loved the guy so much. I've always been happy for her. But since she's out of town for the weekend, I went over to their house this afternoon to check up on things since he just had surgery and is wearing a catheter for now. At first i was just having a general conversation with him and then i started noticing he was getting a little touchy feely when he started rubbing my feet and legs and then he asked me to "come here" like he wanted to give me a kiss on the cheek. I let him do that, but then he tried to turn my face like he wanted to freakin kiss me!! I backed away. And then he asked if he could hold me and i let him for a second. He then said "I'm real sorry. You're a beautiful woman. You're not a little girl anymore." He said he was real sorry and that he got out of hand and asked me to forgive him. I did, but i'm still bothered by it. I'm so shocked because i would have never thought he would do something like this. Right now, my boyfriend is the only one who knows about this. He told me i need to either tell my nana (my grandmother) or tell my parents. He says it's best if they know now than find out about it later. But I'm afraid if i tell nana then it will cause them to have problems and if i tell mom she'll go crazy since it's her mother. Please give me some advice. I'm 21, and I've been hit on by a 73 year old man. :( And no smart-*** answers please. If you do leave them they will get deleted and you will get reported.