Kim, I'm so sorry to hear of your Fathers continuing problems with aspiration pneumonia. Yes, if it continues it will lead to scarring and reduced lung function.
Now , I'm just going to throw this out there. They can fix this by giving him a laryngectomy. This separates the wind pipe - the Trachea, from the oesophagus - the food pipe. Once this is done he can eat and drink all he likes. There would be absolutely no way he could aspirate. Maybe this would be an option?
Tammy

Caregiver/advocate to Husband Kris age 59@ diagnosis
DX Dec '10 SCC BOT T4aN2bM0 HPV+ve.Cisplatin x3 35 IMRT.
PET 6/11 clear.
R) level 2-4 neck dissection 8/1/11 to remove residual node - necrotic with NED
Feb '12 Ca back.. 3/8/12 total glossectomy/laryngectomy/bilat neck dissection/partial pharyngectomy etc. clear margins. All nodes negative for disease. PEG in.
March 2017 - 5 years disease free. Woohoo!