High calorie foods for babies are the best way to encourage weight gain, breast milk has more calories and fat than most other foods. So offer the breast more often.
The next best thing is meat. Meat is well tolerated because the baby has the enzymes to digest animal protein and fat (which is what is in breast milk). Choose meat with some fat. Egg yolks are an awesome source of nutrition for babies, very high in nutrients (zinc, vitamin D, etc) and healthy fats (choline which is needed for brain development). If you can get eggs with DHA (generally free range and/or fed flax) then it is even better. The whites are more allergenic, but more than that they are low in calories and nutrients which is not what baby needs.
The next best thing is meat. Meat is well tolerated because the baby has the enzymes to digest animal protein and fat (which is what is in breast milk). Choose meat with some fat. Egg yolks are an awesome source of nutrition for babies, very high in nutrients (zinc, vitamin D, etc) and healthy fats (choline which is needed for brain development). If you can get eggs with DHA (generally free range and/or fed flax) then it is even better. The whites are more allergenic, but more than that they are low in calories and nutrients which is not what baby needs.