Deputation from IB or FIA is possible only to Special Branch of Police ( SB is the intelligence arm of Police and doesn't wear uniform). There are such instances of deputation from IB.
But it is rare and requires very senior level intervention on your behalf
However, since your brother has exhausted all his css attempts, the only way now to join the department as a uniform personnel is to start his career as ASI or SI.. With consistency and a little luck he may reach SSP rank before his retirement.
Lastly, i would personally recommend that your brother try his luck in PMS which is second only to DMG and PSP and better than every other government job available in Pakistan.