Dr. Greene's Answer
Several issues are raised in your question. Thirteen months is an average age for a Caucasian boy to start walking and anywhere between 9 and 15 months would not be surprising or concerning at all. As to learning to talk, we do get concerned if by 12 months kids don’t babble or imitate sounds, but no words yet can be fine (especially if they are pointing to objects).
But kids who are losing weight, especially if they are also losing developmental milestones, should be checked right away to find out why. It could be from decreased nutrition or from a long list of other possibilities.
Personally, I would not stop breast milk as a way to assess or improve nutrition. A pediatric gastroenterologist is often the most skilled person in issues of feeding and growth.
Also, a developmental pediatrician could be helpful (this is a pediatrician with advanced training in child development). In California, we have a set of regional centers through the Department of Developmental Services where evaluations of children are paid for by the state if they are losing milestones under age 3. See if there is something comparable in your state.