Our troop had a boy very much like this a few yearsa ago. He was well on his way to Eagle, but then lost interest due to a Scoutmaster change. He started showing back up a few months before his 18th birthday, but through his behavior clearly demonstrated he didn't want to be there. A few weeks before his birthday, he handed in his last merit badge and asked for a Scoutmaster Conference. The Scoutmaster, to his credit, didn't sign him off, as he'd seen no leadership or mentoring of other scouts, or any scout spirit at all. His father appealed to our Council, who after some investigation, backed up the Scoutmaster.
This doesn't mean the boy won't be a fine young man, he just isn't an Eagle Scout.
It seems to me that the biggest mistake was the Scoutmaster signing off on the conference. However, if the boy isn't living by the Scout Oath and Law and other Scouts don't look to him as a good example, turn him down. Becoming an Eagle Scout is more than merit badges and camping, it's possessing character that follows you your entire life.