They guy is only 20 yrs old. I really don't think he has come anywhere close to his potential in the half marathon. Given that most marathoners get 4-5 really good marathons in their career, should he not wait another 4 years before giving the marathon a go?
On the other hand the HM is not an Olympic event and I'm sure he wants a medal, so I guess he could step back down to the half after 2008.