RES turtle cannot use his jaw to eat

hello,

please help! our RES is over a yr and been eating fine before today. (nothing in the environment has changed and he has been eating this food for a yr, too.) this morning he opened his mouth and food went in whole and out of his mouth whole; he cannot use his jaw to chew like he used to do. this happened in the morning and then i tried it again in the afternoon. this afternoon he opened his mouth and was having a challenging time getting the food in his mouth and when he did he seemed to choke. our other RES is eating fine. however, i have taken all food out so he does not choke again. his behavior is the same today- hanging out with the other RES, basking, etc. i did notice when he is underwater he does not have any mini bubbles near is nose and mouth like the other RES. what is going on with him? is something caught in his throat? is there anything i can do to help him? thank you.