id tell the "landlord" you want deadbolts instaling on all entrances to your area (doors to the outside AND inside of the main house) either you cover the cost or they do, eitherway you want good strong deadbolts AND only you have the key. (a kid this sneaky i wouldnt put past finding the spare key in their parents "secret" safe spot.
Id talk to the kids father, obviously mother is not a strong enough figure and maybe father can get something done. id agree wit the others in also telling the owner of the house (your landlord) that you will be keeping track of all items stolen and the value will be deducted from your rent check (its amazing how quickly a parent will take action when they know THEY are losing money because of their childs behaviour. you can also supply them with a list of items and their values upon paying each rent check so thye have an itemised list. Make sure you write a formal letter of complaint including the actions you want taking to fix this problem. keep a copy of said letter for your records (that way if things go sour you have documentation)
i would also make it VERY clear that next time you find the boy even trying to pick your locks the police will be called and a formal complaint will be made.
I do agree, if this is the kids behaviour at 14, its only going downhill from here if he keeps getting away with it.