When you have a chance, please watch this inspirational You Tube video - this child is a raw vegan from the start. Granted, she still had breast milk at this age which is absolutely wonderful for the probiotics, immune benefits, fat, etc. but seems that she gets most of her calcium from the green smoothies, and healthy fats from avocado, chia and coconut meat. I'm sure you could use some of these ideas:
https://www.youtube.com/watch?v=RpRuuE6xo6IAs far as "milk" goes probably the doctor is using it as an insurance policy because most kids have 4 food groups: (1) chicken nuggets (2) spaghetti (3) mac & cheese and (4) meatballs - when most kids are so malnourished that they hardly get any vitamin A, folic acid or calcium, then probably milk is their most nutritious food! Soymilk is probably the closest to cow's milk in nutritional value, with a high amount of protein. If a baby can consume legumes, then probably no need for soymilk, but it's just an easy and quick way to get some extra calcium and protein. In India, many babies consume "Kichadi" which is a mix of rice and yellow lentils - complex carbs, protein, and calcium all in 1! My mother made in the pressure cooker, and added some onions, tomatoes, and green peas to the lentils and rice.
For green smoothies, my kids are a little older, but I use Kale, celery, carrot, bananna, blueberries, and just a little bit of soy milk.
As far as the McDougall diet, I believe it requires the modifications of increased fat for kids that age and also purees / smoothies can be an easy way for babies to take in the goodness of greens because of the lack of teeth! Or you could be like Alicia Silverstone and pre-chew the food for your baby
(I personally would feel very uncomfortable about this!)
This is a general article on feeding vegan kids:
http://www.vrg.org/nutshell/kids.php - basically the reason for (plant) milk is that it will squeeze in an extra 300 calories without filling the stomach, since this is a time of rapid growth and fibrous foods can lead to fullness quickly without consuming enough calories, that's why. In the above video, this toddler consumes green smoothies which are calorie dense, as well as coconut meat and avocado, and probably gets (judging from the chubby cheeks) enough calories to sustain a healthy/active lifestyle. White rice and lentils can be eaten to sustain also, however in India they usually add plenty of ghee to fatten a baby up. There are much healthier high calorie fatty foods than ghee!