Missbanana, in answer to your question, this won't be a problem but.....is your little boy's weight still within the weight chart in the blue book? A friend of mine was told her son was underweight too because he wasn't on average, even though he was just over 2kgs when he was born. If his weight is still within the chart then he isn't under weight, he is just under average. Obviously I do not know enough to really advise to do or not, but i don't think that one formula feed a day would really make that much difference. May I suggest that you get another opinion? I also don't know your maternal and child health nurse but i know unfortunately alot advise on their personal opinion as opposed to the documented research.Good luck with it all.