Euthanize biting dog?

My name is Meghan and I am going through a super tough situation and thought maybe you could offer some insight.

I adopted a rescue dog (Eddy) in February of 2011 from our local Humane Society in Revelstoke, BC. I knew from the posting and meeting him that he would be a handful, but I was up for the challenge as I am very active and was looking for an activity partner and wanted to rescue an abused dog. I was also very involved in the search and rescue and noticed that his drive would be good for the CARDA program (Canadian Avalanche Rescue Dog Association). We went through the certification process, but in the end I did not end up validating him, as I wasn't able to feel confident in our ability as an avalanche team. That said, he was still a great companion and exercise buddy. He has always had huge drive. He was neutered very late (after I got him so probably around 1.5 years old). I think he is a lab/shepherd cross but I'm not sure. He shows lab tendencies while eating and shepherd tendencies for everything else. Anyways, as I said, he has always seemed to have a screw loose - barking and biting at the tires when taking him biking/running etc. I tried to calm him down with a shock collar and that seemed to work...but only to an extent.

2 summers ago I was out for a run and a friend of mine biked by...I didn't have a good hold of the leash and Eddy took off with vengeance. He barked at my friend and actually bit him in the shin and broke skin (not deep but still enough to devastate me). I realized the situation was him being overly excited and decided I would try to mitigate this. Later on that summer I was taking him for a run. When I let him off his leash, he got so excited that he actually jumped up on me and bit my bum, broke skin (not deep). Again, I was more shocked than anything else...and after a week or so I had almost forgotten about it (since it was me that he bit). This past summer, some friends took him out mountain biking (I wasn't there), and when one friend tried to reprimand him for barking, he took a large chunk out of his hand that required stitches. My friend was ok with it, saying that he had provoked him and that he was wearing a bark collar so he thought he associated the shock of the collar with my friends hand. Anyways, still no excuse. I decided that from then on that was the last straw, and I would take him out biking with me only, or me and one friend so I could be there to control him.

That seemed to work well. However, a few days ago, my roommate was babysitting a 5 year old boy. Me and Eddy were out for a run and when I got home, I let him off his leash as I normally do. I didn't realize there was a child there and he went roaring up to him. The child started screaming just out of nerves and Eddy lunged at him. No skin was broken, but you could tell there was a red mark where he put his mouth. I feel completely defeated. I feel like I have tried so hard with this dog (and he is so lovely 99% of the time). But I just can't afford to have him bite again. I guess why I'm writing is to see if you think that the humane thing to do would be to put him down. I'm so upset but I just don't know what my options are, I love him so much but I can't keep having this happen and keep making excuses for him as to why he got aggressive. Is there any other solution other than putting him down?