Hi Bluebear,
Neither PIP nor DLA are means-tested and will not be affected by any income or assets.
Everyone who is in receipt of DLA and was under 65 years of age in April 2013 will be 'invited' to claim PIP at some stage (DLA will be stopped if the claimant does not apply in the stipulated time). Even if, by that time, they are over 65 and also over State Pension Age.
PIP, like DLA before it, will continue to be paid for as long as the claimant satisfies the medical criteria - no matter how old they are.
I do hope this information will put your Father's mind at rest