Can an 18 year old boy get in trouble for sleeping with 15 year old girl,if he has mother and fathers permission.

Yes, as that is STATUTORY RAPE.
Parents cannot give permission to break the law!

ANY question on law needs to include your general locale as laws vary by area. In most areas, 15 is under the age of consent. A parent cannot give consent in such a case. Nor can I fathom ANY parent giving permission for a 15 yr old to engage in sexual activity.