Okay I'm glad to hear I'm not putting him in danger at least, although you're right that he most likely knows what he's doing.
Anominus, that's a really good idea, thanks! I hadn't even thought about the need for a key of I'm not around!
I brought up the subject at lunch today with him and asked him whether he had considered using a professional keyholder. He said he had considered it but that he preferred having a live-in keyholder, just in case. He asked if I was requesting payment like a professional and I said no - to be honest it would be something to benefit me without having to be involved physically, but I feel really awkward asking for money - I can't help thinking that the platonic friendship we have would go sour. Besides, he's already paying rent for his room and I already find that to be a point I prefer to avoid discussing since the home situation feels more like friends-as-flatmates than landlady/tenant.
I also brought up the fact that I've been doing my research (thanks guys (

) and that I would be more comfortable if I gave him the key once a month rather than once a week, and that it shouldn't be a problem. He's thinking about it, but we agreed that I would give him the key tonight as previously planned.
I'm only planning on literally handing the key over and accepting it back later, but I'm feeling very apprehensive about the whole thing. I suppose I just need to go through it once - either it will be smooth and no awkwardness, or it will be awkward and I'll just give the keys back...
V