My yorkie, Pasha had the same symptoms as your little one.It turned out to be a calcium oxalate stone. He had surgery 7 weeks ago. The surgery went well, but since then those same old symptoms are back even though he is on antibiotics all this time. He has a good pee and then strains to get out more. He strains so hard that his penis squeaks. I know what you are going through and if your has to have surgery, expect the first night to be pretty tough to see your little one in pain. But by the next day he should be feeling much better. Let's just hope this surgery solves the problem. Three vets have looked at Pasha given him exrays, ultrasound and flushing of his bladder to no avail. It has been and continues to be a horrible experience for both of us. I will pray for your baby that he does well after his surgery. Let me know what happens. I will be thinking of you.
