"He's on to me" means he knows what you are doing. This is when you are doing something either secretively or quietly, and do not want someone to know about it. He's on to me means he found out about what you are doing.

"He's onto me" means "I'm trying to fool him, but he discovered the truth." "I hid my son's birthday presents in my closet, but I noticed today that, mysteriously, my closet door has come open. I think he's onto me." That is, I think he's discovered the presents I was trying to hide.

If you change it to say - 'He's into me' - That means that he has a crush/likes you/fancies you. You could also say: "I'm into him/her" - Meaning you fancy/like/have a crush on and like him/her back