This is terribly distressing for you I'm sure. You should stop force feeding at once as it is making a huge issue out of a problem. Split your dog's feed into four or five tiny meals and allow fifteen minutes of peace and quiet when the dog is on it's own for it to eat. Then remove the food without a word or a fuss. Carry on as usual.
Use tiny pieces of something smelly and tasty like cheese or liver as a reward for very short excersises - your praise for a sit( without the other dog around) needs to be very happy, very high pitched. Anything your dog eats will start to stimulate it's appetite again so even these tiny bits will help.
Mixing a little yogurt or cream cheese with his food may help - but he needs to feel secure, away from you and the pup and that there is a limited time that food will be available. Weigh the food before you give it to him so you can see if he has eaten any by weighing after rather than watching him.
Good luck - please let us know how you get on.