my yellow belly terrapin

I have had my terrapin for about 2 months. He is in a tank with a bigger terrapin the same as him and a common musk which is the same size as him. He has recently stopped eating, swims differently to how he was and bubbles (like foam) come out of his mouth quite often. I have tried to open his month to see if anything has got stuck but cant see also he keeps making a funny noise and swallowing.
Any ideas what I should do. Please?