Welcome to Xmarks!

If you are able to spend about $150, one option would be to visit.
http://www.heritageofscotland.com/Bo...68,product.php
and just go ahead and get him the Kids County Antrim tartan.

If that's not possible you can get an Irish National youth tartan from stillwater kilts for about $16.00
http://stores.channeladvisor.com/Sti...0Tartan%20Kilt
And they ship in about 2-3 days so you can have it in hand very soon.

Good luck and welcome!