Originally Posted by Jimbo1983

Thanks for all the replies!
So it sounds like it should be ok, and while I was looking on ebay last night for fun I saw one of those fluted posts with the insertions marked in larger flutes, I they came on the Centurion Ironman bikes. So those were intended to go below the clamp area which kind of answered my own question.
Someone mentioned scratching, I am a little concerned about that but the seapost isn't pristine anyway so I should be able to polish it out.
We are in a wet area but he's only staying for a day or two so I would pull it back up to my height right after anyway so if it did get wet I could clean and grease it again inmediately.
And as for size I have my seatposts fairly high even on a 63cm frame, he is more like the intended size but his legs are shorter for his height, he'll be fine.

Thanks again