Is he your own or one of the adoptees? If he is your own child, he may be seeking attention in the only way he knows how and may fear the loss of your caring with the influx of four more siblings. If an adoptee, there may be some grieving, loss, and trauma involved. At the very least he may be grieving the loss of his natural family, no matter how bad that living situation may have been. He may be feeling divided loyalties. He may not know what he is feeling!
Assurance on your part and normalizing the experience may help, but a general support group or a grieving group for children his own age may be even better. Check with the school to see if the counseling department offers such groups.
If the lying continues and is serious, I encourage you to seek out a counselor in private practice. The school counselor will give you referrals of professionals.