OK. Here we go.
First, it makes no difference whatsoever that the guy wore 24 forever, if he is going to play O-line, there are rules about numbering. There must be 5 players wearing 50-79 on the O-line, as Highest Point said. These are to identify to the defense who is and who is not eligible. (It's not a requirement that all 5 be interior linemen, but they will not be eligible even if on the end.)
The next part comes with calling WITH the Rules Book and not BY the Rules Book.
If this player is big enough to play DE then he's probably a solid player and would do OK on the O-line. He might even be a viable option as tight end so even though he is intending to play interior O-line, that would not be obvious to the defense. I would also require this player to change his number.
(Side note: Anyone else old enough to remember the numbered pennies guys slipped on to change their numbers for punts, field goals and PATs? That was before the numbering exception was written into the rules.)
This brings me to the second part. I have twice allowed a player to play with an eligible number on the O-line, because the team was clearly not using it as a way to fool the defense. First was a case where a kid playing tackle was probably 430 pounds. He was not going out for a pass. Someone had specially made his jersey and I think the number was 99. There was no jersey for him to change into and the defense was fine with it.
Second time, more recently, a game is 35-0 or something like that in second half. The losing team has a tackle get hurt and they send in a small player wearing 45 to play tackle. Both teams wanted to get the game over and be done. The new tackle was not going out for any passes, but I did announce to the defense that 45 was a tackle and was ineligible. This is just something I did in the situation, he did not "report" or anything like that. There's nothing in high school rules to allow such a thing. Technically that team was committing a foul on every play but it simply wasn't in the interest of anyone or the sport of football to worry about it.