Try putting the canula (long part) in the side of his mouth and have him close his lips around it. That's how I did it in the hospital and it did a good job. I didn't try to stick it down my throat, instead got as much of it into my mouth as possible and then did as I describe above.
Is he using Guaffenasin also? Not sure if I spelled that right, but it's over the counter and really helped me in thinning out the secrections. I didn't have to use a suction machine during my raidation, only during my second surgery.

Good Luck,
Minnie