Perhaps you didn’t see (or didn’t want to see) the warning signs. Or maybe you’re too far into a relationship with one to turn back now. In any case, your significant other is a manchild and now you have to figure out how to cope.
He’s already quite aware of his state and therefore has no desire to change. So anything that comes off as an attempt to parent your manchild will blow up in your face. Attempting to control his behavior through threats or manipulation will work on occasion but they will also cause your frustration to grow.
If he’s willing, suggest some form of counseling. A third party will be able to communicate with him without the emotional load the words would have coming from you. A neutral observer will listen to both sides and may be able to negotiate a mutually beneficial arrangement (i.e., if he refuses to clean, you shouldn’t feel obligated to carry his chores as well – a maid might be a better answer for the harmony of the relationship). Document the arrangement so there are no questions about who does what later.
If he’s unwilling to compromise, you may find yourself in the position of sucking it up and soldiering through. Pick your battles and let the rest go like water off a duck’s back. Of course this “solution” will include arguments when you tire of being the bigger person. At some point both of you will need to analyze how much you truly want this relationship and how hard you are willing to work to maintain it.