First thing to remember is that the PEG has to be pressure rinsed with water after each use, using a syringe or two or water into his stomach. Ensure and other things will harden and clog it up. This is probably what happened. If you have a clogged up tube, you need to try to clear it using hot water and pressure with the syringe, or carbonated water sometimes works like club soda or tonic water. If you can't find that around. a can of coke or something similar might help clear it . If you use that, then rinse the tube with fresh water after the coke. This is likely not backing up because of anything in his stomach. Post again as soon as you have tried pushing water though it with the syringe.