Welcome to the forum. I'm sorry to hear about your problems, maybe with some more information we might be able to shed some light on it. What is the age of your cat? What type of food is he on? Did the vet give you any idea what the urinary tract infection was from? There are several things that can cause it and she should have identified the problem to you. Did she do a urine sample test, and were you able to get the results? If so would you mind sharing the results with us, it would make it easier to get to the bottom of it, and if there was some blood work done the results from that would help also. Have you noticed any blood in his urine?
We have a group on this forum that are quite knowledgeable on urinary tract problems in cats, and these are some of the questions they would like to know.
With him urinating outside the litter box is a sign that the problem hasn't been rectified, and that he is suffering pain. Usually what happens in a case like this is if they are suffering pain when passing urine for some reason they relate it to the litter box, hence the markings all over the house. You are saying he is only passing small amounts. This is also a sign that things are not right, along with the yelling is a sign of pain.
With this being a male cat it can be extremely serious because the urethra tube is small to start with and there is a restriction where it joins the back of the penis. If he is suffering from crystals he could end up with a blockage in the urinary tract, and this has been known to be fatal.
If it was me I think I would be getting in touch with the vets office or seeing if they had an emergency number, and give them a call. Tell them what's going on and see what they suggest. Depending on what is causing the trouble the cat should have been out of pain within 2 or 3 days at the outside, I would think.
Keep us informed.
Animals are such agreeable Friends.
They ask no Questions. They pass no Criticisms.