I am very sorry to hear that Pippin reacts to eating by rearing up, pawing at his mouth and mewing.
All of these actions tell me that it hurts to lap up or chew his food.
That can be related to a gum or tooth root infection, a cracked tooth, a mass under his tongue, on his jaw or gums, or on his palate, or a piece of foreign material stuck in his mouth. Sometimes it is related to pharyngeal polyp or tonsil mass that has gotten very big and is interfering with swallowing making him feel like he is choking when he tries to swallow.
Pippin likely needs to be sedated for a thorough oral exam and radiographs of his tooth roots and jaw.
I would look closely into his mouth if you can to see if you can see redness, ulcers or anything caught. Look up on his hard palate, under his tongue, towards the back of his mouth near his tonsils and soft palate, and check his gums and teeth.
In the meantime you may be able to help him feel better eating by adding warm water to his food to make it softer and easier to chew and swallow.
Please let me know if you have any further questions.