Take a break from all milk for awhile. Right now, he is still very used to the taste of cow's milk, so everything else is going to taste bad in comparison. Wait awhile until he doesn't remember that taste as vividly. Then, choose a new milk and, if it needs to be chocolate, that's fine.
Consult with a doctor though. Many of the alternative milks don't have as many nutrients as cow's milk and aren't really worth drinking. They are good alternatives for putting in cereal or using for other recipes, but don't really equal cow's milk with regard to calcium and vitamins.
As for soy milk, I don't think it's nearly as bad as many people say. My son is 5 and his been drinking it daily since shortly after his first birthday. We haven't seen any negative side effects. It was recommended by his allergist, the nutritionist in his allergist's office, and his pediatrician. He used to drink about 20-24 oz a day, but for the past year or so it's been more like 6-8 oz/day. We use it whenever we bake, in his cereal, etc. He cannot tolerate any cow's milk at all.
Do not switch to lactaid. He is not lactose intolerant nor is his allergy caused by the lactose in his milk. He is allergic to whey and/or casein, the proteins in cow's milk. Many people with cow's milk allergies cannot drink goat's milk either.
If he likes chocolate soy milk, let him have a cup a day. It will provide him with calcium and critical vitamins and will not be a sufficient amount of soy to cause negative side effects.
As another poster mentioned, you can try to slowly convert him from soy to almond by mixing the two together, but before you'd do that, I'd compare the labels and make sure he's getting enough nutrients from the almond milk.