What type of form did your son received? Was it the form 1099-R or any other?
What type of account did he inherited? Was that a tax deferred account - 401k, IRA, etc?
Absolutely - you may come back at the later time.
I suspect that was tax deferred account.
While you are correct that inheritance is not taxable income in the US - and do not need to be reported to the IRS - there is no any amount limit. Please see for reference IRS publication 525 page 33 - http://www.irs.gov/pub/irs-pdf/p525.pdf
The distribution from tax deferred account should be included into taxable income.
My wild guess that he received the form 1099-R - http://www.irs.gov/pub/irs-pdf/f1099r.pdf and that form reports taxable amount in the box 2a; In the box 7 the distribution code should be "4" - Deaths - so that distribution is not subject of 10% penalty.
Please verify if that is correct and let me know if you need any help with reporting.