From the instructions to Form 709: The value of a gift is the fair market value (FMV) of the property on the date the gift is made (valuation date).
I don't think basis comes into play, unless Series H bonds. Your client will report the current value of the bonds as the gift and report all previously accrued interest on his tax return. While he seems to want to avoid filing a gift tax return (even though it is unlikely any gift tax will be due), the bigger problem is adding all that interest to his income in one year, potentially raising his marginal bracket, taxing more Soc Sec, increased Medicare surcharge, etc. You can crunch those numbers for him and perhaps end up advising him to spread the gift over a few years.