Murfie's got a point, it could be a harmless doodle or maybe some form of protection spell. does he practice magic? you can just ask him about it just say 'i was putting away some stuff and i noticed a sticky note, what what it about?' two people in your situation should be honest with each other. if he's spying on you and you're not trusting him at this stage, what'll happen in ten years?
personally, i feel it's just a doodle. if it was a spell, i think it could just be a protection spell, think 'watchful eye'. if the two of you love each other, i don't see why he'd put something like a curse or is spying on you. just talk with him. if you're concerned about your son's safety, give him a protection charm, and put a protection on him to be safe.