Once he came in contact with our 3 year old, things changed

I would agree, wholeheartedly, ours now being 21(The offspring, not the dog) we've got more leeway, but I would have done the same as you, you just can't risk it.
This is how our animal behaviour expert put it:
Your adopted dog is joining a pack and looking for a leader, it's at this point that most behavioural problems occur: In your case, seeking attention because he/she cannot find a natural leader would appear to be the right scenario, but I'm not the expert. How you train is often as important as how you feed, an agressive dog is often so because it has changed to a "designer diet". In the wild, 85% of its diet is chewing bones/carcases, take that away, and you re-introduce the hunter instinct in an unstable/weak personality dog.
When the experts came to us, they spent a whole morning just watching our dogs and our behaviour with them, amongst other things, what they identified was that, the problem was not the rescue dog, but our middle aged bitch (not the wife, lovely women!) not showing leadership.
The moment we started the training regime, the older dog started barking at us and getting very agitated, because, although she was too lazy to do it, she didn't like her leadership role taken away from her.
It's early days yet, but they actually changed behaviour in the day that they were with us, we've got to sustain it.
They recommended a diet, which we've found on the net, composed mainly of crushed bone, and it has, within days, altered the agressiveness on walks.