Cat peeing on wall above litterbox

I rescued 2 feral bonded vessel friend neutered male cats on Valentine's day and one of them is suddenly peeing high above the litterbox while standing in box 6 weeks later...only sometimes.. it's light yellow and a lot of pee. It seems to be behavioral if something scares him or If he's upset I won't give-in to his crying for food. Tonight I gave him food and immediately after eating he went into the litter box, leaned over and peed a bunch against the wall. How do I know if he has a medical issue or behavioral and if behavioral how do I stop it? Again it's only some times... he's a very happy cuddly cat other than that..but tiny bit skittish He's never shaked his tail until last night at the very end of peeing but that was during a regular low pee that got higher and higher at the end..my senior cat with FLUTD did that when he occasionally needed 1 prazosin   How do you know if it's a uti or flutd or behavioral? He's terrified of hoping to vet and I financially can't pay $150 if it's just behavioral...and additional stress would just cause more wall peeing

There is no such thing as 'just behavioural' - you sound like someone who has an awareness / experience of male feline " UTI " a few years ago, but veterinary understanding has changed.  Back then, we assumed that thesse cats had infections which caused them to struggle to urinate - indeed, cat urine caught in the environment often had bacteria in it, which we thought explained the signs.  Over recent years, after lots o new research, our (vets) perspective on this has changed - and thereby, treatment is becoming more successful.

The fact is that blocked urethras and dysuria in male cats is rarely caused by infection - more often it is caused by social stress or competition, and two male cats - especially sharing one or two litter trays - might be a typical starting point.  As you may know, urinary problems in cats can go on to cause blockages and pain, so now is a very good time to get such behavioural issues sorted out.  I can provide some links

In 2001 Sian Tranter wrote a blog about cystitis in cats that you can search for on our blog page.

Sophie James 'How to make my cat feel happy' might also have some relevance.

It is worth explaining that even if your cat doesn't appear to be showing signs of cystitis at the moment, the urinary sign that you are seeing may suggest some social urinary stress and the concern is that one leads to the other.   We would reccommend several litter trays - a minimum of one per cat plus a spare - and opportunities for each cat to get to food and to a litter tray without crossing the other cats 'territory,' if you can work out where in the house / room that is.  We would certainly reccommend going to talk the situation through with your vet (take a fresh urine sample if you can get one) even if it does seem to be 'just behvioural' at the moment, because behavioural patterns are so interconnected with cystitis.