DS is 13 months old and I have recently switched to cow's milk from formula. I still give him formula; I keep the ratio (50:50). I read a lot of things on the internet about formula having certain esssential nutrients like iron which cow's milk may not provide.

Any suggestions on this? DS is a little underweight and doesn't eat solid that well. Milk/Formula is still a major source of nourishment for him. When would it be good to switch to cow's milk completely.

Thanks

Not until he is eating a reasonable variety of solids, certainly.

I wouldn't switch to cow's milk until he's eating a sufficient amount of solids to sustain his growth and development. I'd use a stage 2 formula and work closely with his ped in regards to what to feed him and when to go to whole milk.