Client was released from prison 9 months ago, was not aware that he was entitled to dla so did not tell them when released and only now have dla been told.

Will his dla be paid from date of release or will it be less due to long delay in informing change of circs?

Claimants are disqualified from receiving DLA when they are imprisoned. Disqualification only affects the payability of the benefit not entitlement to that benefit. This means that when the conditions supporting the disqualification no longer apply, payment of benefit can be resumed providing all the conditions of entitlement remain satisfied. Unless entitlement has been terminated during the period of imprisonment a claimant should not be required to make a new claim to benefit on release.

The above is taken from the DWP decision makers guide; so I would infer from this he should receive full arrears of the reinstated DLA, unless his care/mobility conditions have changed meantime.