Hello WorriedMom - thanks for visiting.
As you say, cross dressing of this type is a common enough way of exploring youthful sexuality. Womens’ underclothing can be ‘mysterious’ and therefore exciting, and that can be part of the cause.
It is usually linked to the pleasure of masturbation, and if the person becomes to fixated on it, it can cause failure of an appropriate sexual response to the opposite gender in later life in the absence of the desired clothing.
It is not, however, a sign of latent homosexuality.
Part of your son’s unwillingness to discuss this will stem from the fact that he does not want to be thought of as a ‘sissy’ (a teenage boy’s ultimate nightmare!) and because of a likely link to masturbation. Keep working at keeping communication open, and try to treat it lightly.
In general, it is regarded as harmless, and tends to disappear as sexual issues get sorted out, but if it continues into late adolescence, you might wish to have him evaluated by a psychiatrist.