The answer depends on several factors, including his age, religion, family values, and comfort level. If you follow a religion, especially Islam, you should not let your adult son have his girlfriend sleep over in his room. According to Islamic law, a relationship with an unmarried girl is considered haram (forbidden). After death, Allah will punish such haram activities. Therefore, according to Islamic law, a girl cannot sleep over in her boyfriend’s room until they are married.
However, if both the boy and the girl are still litttle children who are not physically mature enough to produce semen and are not familiar with sexual activity, either physically or mentally, they may stay in the same room, as they would still be considered little kids.
If you don’t follow any religion and live in a culture where the free mixing of adult girls and boys is allowed, it may be possible, but I personally don’t recommend it.