jeezo..does sound pretty serious but could be such a large list of things,
how long have you had him again to him getting ill ??
he's quite well built so doesnt look like he's had a chronic illness.
sounds like he's anaemic so possibly losing blood somewhere or not making enough rbcs ??
where did the vet reckon there was fluid ?? his chest or abdomen...?? i am presuming his chest
i could just stab in the dark really at this stage and suggest any of the infective diseases, trauma, tumour, heart failure, might have though warfarin but think youve had him too long for that...often with a diaphragmatic hernia it can be worse after they hve eaten..obvious as the stomach/gi tract full so more FB in the thorax
I'd say if he is getting worse then he might need to go in tonight but i know i dont need to tell you that
sorry really wished i could have helped more on this one .