I'm not sure what pain meds he is on but some may be short acting(usually 4 hours) so as those wear off he may be waking up.
There are pain meds that are longer acting and less disruptive to sleep patterns.
The MD may also be able to help with the obstruction relieving the breathing. The discomfort should be stressed to the Dr.
Any good MD is interested in treating the whole patient so mentioning the sleep issues is important.
There are liquid supplements that your dad can drink to help him with his nutritional balance as well. You can look at the amazon link here and look up Carnation 560.
In our case, the ins pays for the supplements. So, we had to order it through the MD as well.