What you have just noticed in your male friend is not something new. Most women have this complaint about their male friends.
Scientists have found evidence which suggests that that the first thing a man notices about a woman is her chest or her waist rather than the eyes or face. They concluded that the reason for this was that the male inspects potential mates for child bearing.
Since we see this behavior on media and television, in daily life too, man begins to follow what he sees.
The researchers arrived at the conclusion after attaching eye-tracking devices to 36 men that measured how long they looked at something. The experiment was not just limited to men, 29 females were also tested.
The participants were shown pictures of ten women with three body shapes: curvy, thin or medium build.
The researchers found that men gazed longer at women's bodies than their faces, particularly those with larger breasts, narrower waists and bigger hips. Also, women looked at other women's breasts and hips more than their faces.
So, you should not be worried. It could be just a reflexive action. There may be nothing in the mind of your admirer. If you really don’t like his stare, you can tell him about it or turn away every time he looks He will get the hint.