Sliding a tube back and forth in an ig's mouth to get farther down the esophagus will cause pain within a few days. Instead, pry his mouth open and deposit about one cc of water at a time
WWhen you want to open his mouth, there are a few steps leading up to it.
Hold him in your arms until he calms down. Wear hard smooth clothing like denim. Make sure his toenail tips are trimmed. Gently take hold of his dewlap, and slowly start pulling until his jaws get tired and his mouth opens.
Pulling hard on the dewlap can panic them and reduce the trust you have built.