I am very glad to hear that you are having him scanned today. Hopefully, they will also take a sample of the fluid present. My concern here is that pale gums + fluid retention do raise some serious concerns. We can see them when there is an internal bleed (though this sounds to be more a chronic issue and not a sudden one). Also, and my major concerns here, we can see these signs with underlying heart disease
, liver disease, and when an animal has a deficiency in blood proteins (often related to chronic diarrhea or protein loss in urine).
Otherwise, we can see the distended abdomen with fluid in cases where there is a bacterial infection (peritonitis), though that sounds less likely here. As well, we do see diseases of the lymphatics and also cancer cause fluid to build up in the abdomen.
So, those would be our concerns here for what may be triggering his signs. And again a ultrasound is highly advisable here since it is a good non-invasive way to see what is going on in that abdomen (to see the fluid, the state of his organs, and see if there are any growths
present). By using this tool and checking the fluid type, your vet will be able to greatly narrow our concerns for your lad. Depending on which issue is present, they can then advise you on his prognosis and the best way to help him.
Please take care,