Stepmom question

So, I have a question about my stepson who just turned 5 years old. First a little background, his biological mother really doesn't want to be around him and frankly doesn't take good care of him; because of our jobs (mine and my SO's) right now we can't have full custody. Recently I started to notice a new behaviour, He wants to kiss me in the mouth, he tries to touch my boobs like he is doing something else... and he wants to somehow touch my genitals in general... I don't have kids of my own so I'm lost. I feel concerned that this is not a typical behaviour for a 5 year old that is exploring naturally and that it is a sign of something else, maybe he feels neglected by his mother?. Since his mother is not really there, we won't get an answer besides I don't know what you are talking about. So, to all the moms out there. Is this normal? What does it mean?... If there is a problem how can I help him? Thank you so much!