He's allowed £1000 interest per year from savings accounts. If he has more interest than that, it's taxable. I don't think it changes your tax cade - it's added to your income. But if you owe tax from last year (because savings rates went up for example), they reduce your tax code so the tax gets paid back.
(Originally, your savings interest was paid after tax, but when the put the £1000 allowance in place, this would have got complicated, so now interest is paid gross, ie without the tax taken off.)
And I presume you're taking account of the fact that the tax code is the allowance divided by 10, so a tax code of 400 means an allowance of 4000
Finally, up to now they haven't taken the tax from the state pension - for simplicity they've taken it all from the private pension. So a state pension of £8500, for example, would eat up £8500 of the allowance, leaving him with £4000 allowance to set against his private pension, so his code would be 400.
(whereas he might expect a tax code of 1257 to reflect his personal allowance)
I say "up to now", because for the first time the state pension can be more than the taxable allowance, so for people who have only the state pension, they're having to grab the tax from that.