Most Helpful Guy
My parents did the same thing. They are having a hard time seeing him leave the nest. Mothers usually have a harder time because no matter how old he is, he is still her baby. Because he is staying with you, his parents feel like they are being dumped and you are more important to him and they feel left out and second place. They just need to realize this is part of life and eventually he will move on and move out of the house. They will realize that it doesn't mean he loves them any less and staying with you doesn't mean he is ruining his life somehow. Try not to take it personally since your relationship is still very new. They probably don't know you very well yet and think sleeping over is too soon and not proper either.
Most Helpful Girl
maybe because they think its too early for their son to spend all of his time with a girl he's been with 3 months only.