Percocet is a combination of oxycodone and acetaminophen. Both should be completely out of his system within about 20 hours if his kidneys are functioning properly. Too much acetaminophen, however, can cause kidney and/or liver problems – particularly if he already had problems in those areas to begin with. Generally speaking, the amount of acetaminophen he is getting with only one Percocet a day shouldn't be too much unless he is taking other medications (either prescription or over-the-counter) that also contain acetaminophen.
I'm not a pharmacist – I'm just going by the information in my drug reference. You might want to call your dad's pharmacist and ask him/her that question to be sure. Most pharmacists are happy to answer any medication-related questions.