How do you keep the syringe in place during gravity feeding for PEG tube?

You hold it in place with your hands. If it is too hard to do over a few minutes request an instruction of other modes of giving feedings. You can get a bag, but they need to be changed frequently and will make him more prone toward infection, and they are costly. As Youtube has everything, there is likely a film out there on administration of peg feedings by syringe. Give that a try. Do you have a visiting nurse? That will help as well. I found this online. Looks pretty good.
https://www.youtube.com/watch?v=wcnGOIX3tuw