There may be a special statute that applies (although I am not aware of one), or the limitations period may be extended due to his (> 4 month) absences from the country. So don't assume that the statute of limitations has run.

Your son shouldn't have blown off the college back in 2004. I expect that he signed a contract obligating him to pay any amounts not reimbursed by the National Guard. Perhaps he can follow up now and get the Guard to pay what they owe.