I am sorry you are both going through this, from experience it is difficult to deal with especially if you are going through a hard time yourself for various reasons.
As our family fosters cats we have experienced all sorts of cats with various problems with regard to peeing outside their litter tray and so below are a few suggestions that might/might not be of help.
I would second the suggestion of getting him checked again by the vets for a UTI, Kidney disease or diabetes. He could still have a UTI.
Get yourself a UV light Cat Urine Detector as this will show up exactly where your cat has peed. You may find you have missed some areas of pee. You need to use an enzymatic cleaner on the area to get rid of the smell. As someone has suggested already try using Natures Miracle on the area - they do a wide range of products especially for cat urine if you go to their website you can see what is available . If you can't get hold of this, a solution of 50% white vinegar (or apple cider vinegar) and 50% cold water left on the area for a few minutes then blotted up before using a solution of dilute washing up liquid to wash the area. You can also use Bicarbonate of soda sprinkled on and left for a while before hoovering up helps with the smell. (You may need to repeat several times) Also if it is carpeted area the pee might have gone through to the underlay and the flooring underneath so he may still be smelling it, so it might be worth checking in case you need to clean this area as well.
Just one thought, could the cat outside have marked/peed outside and your cat maybe still smell this - if so this would continue to put stress on your cat. I would give the outside area a good wash with a solution of white vinegar and water followed by washing up liquid (including walls/windows/patio) and see if this helps.
I know my Mum's cat suddenly started peeing by the front door and it was only when a friend came round on a sunny day and noticed that the doorstep/front door was wet on the outside that we realised that a neighbourhood cat that bullies all the neighbours cats was marking/peeing on the outside which was causing stress to my Mum's cat hence the reason for her peeing on the inside!!
Another possibility is how many litter boxes does your cat have and how often do you clean them? It is highly recommended that you have one litter box per cat plus a spare (especially him being an indoor cat). I know many cats that don't like using dirty (or what they think is dirty) litter boxes and will go elsewhere.
Also whilst taking about litter boxes - have you changed the type of litter that you use recently? It's just this can upset some cats and so cause them to go elsewhere.
Catsan - the cat litter manufacturers have a page on their website that gives useful hints on how to get rid of cat pee and the smell.
If you go onto YouTube, Jackson Galaxy has various videos giving advice on how to deal with cats peeing in the wrong area.
Sorry for the long post but I hope some of the suggestions above help. It is hard when cats start misbehaving like this especially when it is so out of character but hopefully you will get to the bottom of what is causing this and have a happy you, stress free cat and sweet smelling home again soon. X