Check with the Town Hall to see if he even has a certificate of occupancy to rent the "apartment" - since it is above a garage, probably, but can he move into garage and use your utilities? NO. if property is listed as a "single family" and your lease simply refers to the address, then an argument can be made that the two of you, and you alone have tenancy and thus possessory rights to the property and his coming, sleeping, using your water and electric are all violations of the lease and potentially criminal actions including trespass and larceny.
Your lease is key. Please contact a lawyer or the housing court sometimes has advocates available to answer your inquiries. Best bet is to send written notice of his breach of warranty of habitability and interference with your quiet enjoyment of the premises. Withhold rent under the theory of constructive eviction, stay as long as possible, save the money and find a new place. Screw this guy - he sounds like a complete moron who will lose in court if what you say is true and should be arrested for converting your utilities to his use.