Many Weims steal food from counters. I'm not sure that's not what happened. In that, you KNOW chocolate can be poisonous/toxic - I have to wonder WHY it was left out with easy (it turns out) access?
WHY would you give no thought to it or to leaving it OUT, yet leave the animals with easy ability to get to it? Had you created the dog, at least he would not be sick? I question whether a cat would have eaten any of the chocolate.
It is also unfortunate you thought some had been eaten, but did NOTHING about it. It's too late now, other than the outcome. Vomiting & having to clean it up, is the least of the possibilities. Milk chocolate is not nearly as dangerous as cooking or dark chocolate. It can cause heartbeat irregularities (see full list below).
While he may not have eaten enough to be fatal (too bad you have no idea how much) you'll have to ride it out. Had you seen a vet, ASAP, the vet could have pumped his stomach, given him charcoal, etc. Theobromine & Methylxanthines are the active toxic ingredients. Caffeine and theobromine have an effect on animals similar to that on people. They increase the breathing and heart rate, sometimes causing irregular beating of the heart. They cause restlessness because of the changes in calcium and energy sources at the cellular level. Caffeine also directly stimulates the myocardium and central