My fiance's son clings to her so much?

I am engaged and my fiance and I each have kids. She has a 4 year old son and I have two boys who are 6 and 3. My kids love her and her's loves me the same. We are all close. Her son though is very touchy with her, nothing inappropriate. He manages to sneak into bed with her almost every night after we fall asleep and then in the morning he will wrap his whole body around her and kiss her over and over. He is about to be 5. My kids are affectionate and can be very sweet but it almost seems over the top. Although I know he and I have bonded and we are close I almost feel like he is competing with me for her attention. I don't ever show it bothers me and I have just kissed her and acted liked he wasn't there before, but what would you do/think if your significant other's child was so touchy and clingy?