Sorry to hear Oscar isn't feeling well. I had a Yorkie in this week that also was feeling poorly, so I hope I can help.
Can I get a little more information about Oscar? Why did you originally take him to the vet? Did he feel warm to you or was it because he wasn't moving around much? What was his temperature and what antibiotics did they prescribe? Did they do any bloodwork?
Is he jumping up on furniture normally (providing he is allowed to do that!), or going up/down stairs ok? Does he try to jump up on your leg? Any vomiting or diarrhea?
Do you remember what his temperature was?
Ok. There are a few things I would be thinking about. The first is back pain. The fact that he doesn't try to jump on you or whines to get off your bed makes me think he is painful somewhere. Although he is young, disk problems can certainly occur and Yorkies can have neck issues at a young age. You could try to gently move his head up and down and side to side to see if he whines or cries with that.
With his posture of having his butt a bit up in the air, I also think about GI pain. If he is eating pretty normally, having normal bowel movements, no vomiting, not gassy, etc, then GI problems seem less likely, but still a consideration. He is still a young dog and it is always possible he may have eaten something he shouldn't have.
Anal glands are a possibility if there is pain in the back end. Anal glands are located on either side of the anus and normal express a small amount of dark fluid with each bowel movement. They can get impacted and not empty correctly. Then they get swollen and painful. Some dogs will scoot their butt on the floor or lick excessively under their tail to try and get them to empty. But every dog responds a little differently to full anal glands--some just don't know what to do and just act generally "not right."
No, luxating patellas ("trick" knee caps) are unlikely to be an issue.
If he did have a fever, then it would be nice to do some bloodwork to see if his white blood cells are increased, check a urine sample to make sure it isn't a urinary tract infection. Bloodwork to look at his electrolytes (sodium & potassium), albumin, liver, and maybe to check a resting cortisol level would also be good.
Anal glands? You can run your thumb & index finger down either side of his anus and see if you feel any lumps under the skin. The glands are about 4 o'clock and 8 o'clock when looking at the anus. Not feeling anything doesn't necessarily mean anything, but if you feel a lump, then I'd have the vet check them.
Yes, if antibiotics didn't seem to help, I definitely think bloodwork is the next step. X-rays might be good, too, to look at his stomach/intestines as well as back.
So he was fine this morning and at the vet, but since you brought him home he isn't acting well again? Was he 100% better while on the antibiotics?
I would definitely give your vet a call this afternoon (ok, it is afternoon here, but I bet it isn't there!) or when the vet is next open. I think there is more going on than just a bacterial infection somewhere and I'd like to get to the bottom of it! Be sure to tell your vet that Oscar hasn't been completely better even while on the antibiotics. I'd do the bloodwork.
I feel bad for him, too! If you have a thermometer at home, it would be good to check his rectally. Put a little lube or petroleum jelly on the end and insert it about an inch into his rectum. A normal dog temp is 101.5F.
You're welcome. I hope little Oscar feels better soon!