Strange behavior

My almost 6 year old grandson has been exhibiting some odd behavior. About a year ago he became quite interested in diapers after being fully potty trained. He wants to wear them and keeps mentioning them. If he can get his hands on one he will sneak off to his room and put it on, even though they don't fit him. At other times he makes his own out of toilet paper or uses one of his blankets. He says he likes how soft they are. When he was finished potty training he was all excited to get big boy underwear. He picked out the ones he wanted, and for a while he was obsessed with them. He would try on every pair and sometimes would put all of them on at the same time. Just recently he began getting his moms underwear out of her drawer and putting them on ( in his room). When he first began discovering his body and liked the feeling touching it gave him, he was told it was ok but it was a private thing, he calls it "alone time" and it seems that it is what he uses these things for. Is this cause for concern?