The Disability Living Allowance your brother receives isn't taxable. The current rate of state pension is £113.10 per week which equates to £5,881 annually. Assuming the private pension is £428 per month, that equates to £5,136 annually. His total income is, therefore, £11,017.
If your brother was born after 5 April 1948, he is entitled to a personal allowance of £10,000 which would mean that his taxable income for 2014/15 will be £1,017 (£11,017 - £10,000) on which he would pay tax at 20% (£203.40 annually or £16.95 per month).
Your brother should call the tax office on the number here
and ask them to review the tax code being operated against his private pension and to change it while he is on the phone, if necessary. If they say they cannot amend it, he should ask for the reason and you can come back here and tell me.
I hope this helps but let me know if you have any further questions.