My cat has been meowing one or two times like a kitten while he goes to the bathroom…

I don't believe I would consider this normal; especially if it is a change for him. I would try to figure out if he is doing this when he is urinating or defecating. If it is while urinating, a urine sample should be checked if it has not already been done. Some cats will vocalize with feline lower urinary tract disease. If the meowing is with his urination, you may also want to rule out a bladder stone through an x-ray. If it happens during defecation, I'd want to be sure he's not constipated or having anal gland issues. This is always tricky, symptoms can come and go and conditions like lower urinary tract disease or a bladder stone don't show up on a normal physical exam and need more investigating. Even with this said, you can talk with your veterinarian further. Certainly if he felt your cat was fine, he likely is. But if you're still concerned or the issue is ongoing, it may be worthwhile to pursue it further.