Just create a guest PC account and don't let him use your account. That way you can limit all programs and files he can and can't access.
This could work well, you could just password your account and he couldn't access it, then you're free of worry
Can't you just log out of your steam account when he's there? I'm assuming, from what you said, that the games you allow him to play aren't on your Steam account?
Another suggestion, in case you can't find what you're looking for, you can try backing up your saves before he arrives, it's kind of tedious I know but it's what I have to do for Minecraft when my nephew visits.
Logging out of Steam could also work, but you would need to make sure you logged out every time he uses the computer, so I would stick with the first solution myself, but it's up to you and it does depend on the circumstances
Processor AMD FX-6100 Hex-core, 3.3GHz > Intel Core 2 Duo Dual-core, 1.6GHz
Memory 16GB Corsair Vengeance DDR3 1600MHz > Crucial 3GB DDR3 1066MHz
Graphics Radeon HD7770 + Radeon HD5570 > Intel 3000 Integrated Graphics
Motherboard Gigabyte GA-970A-DS3 Socket AM3+ > OEM Latitude XT2 Motherboard Socket P
Storage Seagate Barracuda 2TB SATA 7200.14 > Samsung 64GB SSD SATA 3Gbps
Power Supply Cooler Master Elite ATX 500W > OEM Dell Power Supply 90W
Case Zalman Z11 Plus 4x 120mm fans, 3x 80mm > OEM Dell Latitude XT Case
OnePlus One 64GB > 4G LTE, 2.5GHz Snapdragon 801, 3GB LPDDR3, Adreno 330 GPU, 13MP camera