I joined a closed group on Facebook that specializes in taking care of and giving advice on baby rats and mice, and the one person in particular I've been talking to on there says that if he's in a tank set up to mimic the wild, with a place to hide and minimal interaction, his instincts should kick in just fine once he's released. They also said that soy milk does not contain enough nutrients, but otherwise did not rule it out as a milk substitute. They said formula or kmr would be ideal, but they did not say that soy milk was not okay. Besides, this little guy's instincts seem pretty intact - he's afraid of loud noises and fast movements, and he burrows into the hay.
I'm going to wait before adding solid foods, and when I do, I've been told milk soaked cereal is best, so that he learns to eat food, but still gets the milk.
As I said before, I only have the one. His brother passed a couple nights ago. I also live in a very small town, so finding an experience rat owner to take on this baby is nigh impossible.