Epigastric discomfort and constant belching, weight loss, blood test came back negative for h. Pylori? ...What else could be? *young patient.

Epigastric pain. Probably warrants a visit to a gastroenterologist. The discomfort and belching may just be reflux that might respond to a trial of an over the counter medication like otc Prilosec or similar. The weight loss is worrisome and along with the other symptoms needs to be further evaluated.
Dysbiosis. H Pylori although important is not the only cause for stomach problems. Often stomach problems are caused by imbalance to gut lining. Addressing diet, supplements, perhaps food allergies can do a lot to improve the problem
https://www.thedocstu.com/blog/gi-mucosal-immunity.