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Usually I only try to induce vomiting if it has been an hour or less. You still could try it, but you either need peroxide, or to take him to an emergency hospital to induce vomiting with apomorphine.
An emergency clinic might be able to remove the towel with an endoscope, if it is still in the stomach and has not passed into the intestines.
Was the towel chewed into pieces? If it was all one piece, there is a greater chance of it causing a blockage.
The best thing would be to go to emergency now, to induce vomiting. If that is not an option, then all you can do is wait and see if it passes. That could take up to a week. If it causes a blockage he will stop eating, and start to vomit.
It could be tonight, it could be days from now. Keep monitoring appetite, and gum color. If he stops eating, or if he vomits, or if his gums look pale rather than bright "bubble gum" pink, he needs to be seen by your veterinarian right away.
It might be less expensive to go to emergency today, induce vomiting and have the towel come up, than having abdominal surgery in a few days. Here's hoping it passes; it always amazes me what some dogs eat and pass out a week later, without being sick.
Please let me know if this is helpful and what else I can answer.