I read this book in late 90's, its young adult. All I remember is it's a about a young boy who has a key (I think? ) and when he opens any door he goes into the past and sees himself when he was very small with his mother. I thought that the title was one word and for some reason "If" I thought was the name, but when I searched for it I didn't recognise it.

Please if you think you might have a clue as to what the bok is called, I would be so grateful.