Co-amoxiclav underdosage effects

I have a 5 year old cat whom I've taken the vet. He was given co-amoxiclav because the vet said he has bacterial infection. For 14 days my cat is to be give 1ml 3x a day. Now sometimes my cat would spits the medicine. I thought he would swallow it but after awhile he'd just sort of drool it out. I'm worried if he's underdosed because of this. Will it affect the bacteria's resistance?

Yes, he needs to get the full dosage or it might not be effective.  You could talk to your Vet about possibly getting it in pill form, if you think that might be better.