Fluid buildup

[Gokcin]

Good afternoon,
I have a 10 year old shih tzu-bishon mix dog Kenobi. Kenobi had a history of heart murmur and a slight heart enlargement. Past Jan he went through surgery to remove mass from the front of his heart which ended up being thymic carcinoma. Everything was going well. Went for his check up in march and there were no mass but a tiny fluid build up around his staples.
We know during surgery they had to remove edge of his one lung and a nerve to clean the tumor. But 1week after his follow up check up we noticed a fluid buildup under his chin and his chest. He is 15.8 pounds and they have been performing a drain every other day and taking out approx. 300 ml of fluid each time. We had the fluid tested and it came back with no cancer cells or infection. But fluid keeps building up to a point that he is having quite trouble breathing but no attitude change. Still eating,going to bathroom and wagging his tail. We are using metacam but seems like no help.

Vets here are not able to diagnose or slow down the build up speed. Any opinion you can provide is greatly appreciated. Me and my husband do not have any kids and Kenobi is like my baby. Please help!!!