Bless you, OP, for your kindness, generosity, and attempt to understand what to you are weird beliefs & practices.
If the next ritual occasion for this young man is spring equinox, he's some sort of NeoPagan. Were I in his shoes, I would not be comfortable substituting battery-operated 'candles' for real ones in my ceremonies, as the presence of live flame has deep spiritual meaning for us, as it does for many other religions. If you're worried about fire, I suggest requiring him to use enclosed candles rather than tapers, and (a safety trick I use) setting the candle glass in a wide, shallow dish of water. Should the glass break or the candle get knocked over, the water quenches it instantly and also prevents hot wax running all over the furniture.
But I also understand your reluctance to have Pagan rituals going on in a Christian household. Not exactly compatible. Banning his practices outright might drive him away and into possibly risky lifestyles, but you don't have to cave completely. Your willingness to have him do them outdoors -- even in a Canadian winter -- is commendably broad-minded, and the suggestions others have made, such as building a snow shelter, are brilliant, especially considering that Paganism is largely about nature. Instead of a candle, a big fire in the firepit is even more traditionally Pagan, and has the added bonus of keeping him warm.
You may also, though, have concerns about your own kids being curious and attracted-to your guest's exotic practices. Most kids who first encounter Pagan rituals are fascinated, not so much for spiritual or religious reasons, as because it's just so *different.* You have only two options, I'm afraid: to keep them from seeing/smelling/hearing what he's doing, or to come up with some satisfactory explanation that'll quell their curiosity. A frank talk with the young man, as others have suggested, may help a lot, as -- if he's smart and at all sensitive -- he could have some good ideas in that regard.
I hope this helps, and again, bless you for your kindness to a young man in a bad situation.