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 Pentium M 2.0GHz Single-core, 2.0GHz
Memory 8GB Corsair Vengeance DDR3 1600MHz > Hyinx 1.5GB DDR2 667MHz
Graphics Radeon HD7770 + Radeon HD5570 > ATi Mobility Radeon X600 400MHz Core
Motherboard Gigabyte GA-970A-DS3 Socket AM3+ > OEM Latitude D810 Motherboard Socket 479
Storage Seagate Barracuda 2TB SATA 7200.14 > Western Digital 80GB IDE 5400RPM
Power Supply Cooler Master Elite ATX 500W > OEM Dell Power Supply 90W
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