Kim , aspiration is caused by a non functioning epiglottis. This will let any fluid slide on down into the lungs instead of going down the oesophagus
This also includes any saliva. Fluid in the mouth with teeth brushing. Just swilling the mouth out with fluid will mean some goes into the lungs.
I am so sorry that your Dad is dealing with this.
Have his team discussed this issue with you both and more importantly have they offered any suggestions or options to deal with it?
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!