Welcome to the site.
I am Dr. Kaushik and i believe i can help you with your problem.
I will like to ask that whether the child in question expresses his wishes to be dressed in that way or is he being made to wear female clothes as per the wishes of the parents just for the sake of fun?
I will get back to you once you reply to my query.
Well usually such kind of fun activity is taken upon by almost all the parents and it usually does not have any effect on the child's future as far as issue of gender identity disorder is concerned but I believe you should consider the fact that during these formative years of learning ( the age that you have mentioned ) children begin to explore their sexuality and also tend to become curious about their gender , so by doing such an activity the child may get confused about his sexuality when he has just started to appreciate his gender and differentiate it with the opposite one which at times in those who are vulnerable for gender identity disorder may precipitate the same in future.
So putting it in simple words although it usually does not influence a child who has identified his gender to get driven towards a conundrum regarding his sexuality but purists believe that these practices should be best avoided to allow the child become more comfortable in his natural gender.
I hope this helps you to see the bigger picture.
Wish you and your son all the best.