Runny Fudge?

My son helped me make a batch of fudge. I told him to put the chocolate chips and stick of butter in a large mixing bowl and melt it in the microwave. I then put ten marshmallows and two cups of sugar in a heavy skillet and asked him to open a can of evaporated milk. Since I only had tall cans I told him to only put half of can in there.

I put the sugar in the skillet and turned to get the milk, he had poured it in with the chips and butter. I tolded him we had to have the milk to melt the sugar and marshmallows. It doesn't take very long to cook the sugar once it starts bubbling. It's the first time I've cooked fudge on an electric stove. We let it cook about 15 minutes. Usually it cooks in 4 minutes. I told him to go ahead and pour it over the chips and butter. It so happened, I only needed 6ozs of milk, but he poured the whole 12 ozs in the bowl of chips and butter So it runs like syrup. How do I get it to set up?