Any idea what this behavior could be?

So my 5.5 yr old male experienced about 60g of weight loss in the last few weeks or so (he's about 960g now). This is with eating and drinking normally. He is also more vocal than than usual. His poops seem to be a lot smaller than usual. I took him to the vet, and he said to give some critical care for two weeks to see if weight came back. Said teeth were fine. We tried to get a blood sample and that was unsuccessful even with him being under light anesthesia. He also suggested an x ray the next time. But he seemed to be guessing as to what it maybe at times.

Today I noticed he seems to be hesitating to eat lettuce or something here. He eventually started eating, but it took him quite some time to chew a small piece even. Still the vet said his teeth were fine. Any of you guys ever see this behavior and know what I should ask the vet next time I go in a few weeks? I also tried giving him some sherwood digestive and urinary tablets to see if he was backed up or had some sludge etc. Let me know what you think. Thanks!