You can gently pull their bottom lip down and slide a syringe in. Once it's in on the side, move (while still between the lips) towards the snout. May take a couple tries, but they should open up and start chewing on the end of the syringe. Now slowly squirt the food in, down towards their tongue. It's very important he does not aspirate.
It helps to offer food from the side rather than straight down too. That way if the plunger slips it doesn't push a bunch of food down the back of his mouth at once which can cause it to go into his lungs by accident.