We had the same problem with our old lady and an old boy before her. And ditto thank heaven for simple solution, but the wee corrodes everything ....
Anyway, my theories with the benefit of hindsight, and solutions such as they are ...
I'm assuming he is a neuter if he has been living inside with you and another cat... if he is a full male and in double figures then you aren't going to win ... he is going to have to go into a run.
Q1 Is Alfie your boss cat - or was he the boss cat until your mum's cats arrived? Is he dealing with a challenger? Because that would be a major reason for spraying. In which case you are going to have to assert a pecking order with your smell - I found washing down all the spraying places with a very smelly (not horrible but a bit wow!)disinfectant, and using the same smelly disinfectant on the people loos helped when we had a boy challenge situation. On that occasion the male cat was challenging my husband for top cat!! But we had just introduced a girl cat to the household ....
With our spraying old girl, I think she was getting frightened long before I realised
... and she was marking to assert herself but also to give herself confidence. She was the oldest and the dominant cat (to the end of her days) and all had 24/7 access to outside. We were also decorating and doing up the house so things kept moving around.
Q2 - how many litter trays do you have for 5 cats? I think one of the reasons she was spraying because she wasn't confident with the one she shared with the two others - and wasn't confident outside. Adding an extra covered tray elsewhere that was easy for her to get into helped. I think it was less threatening cos there were options and she was safe from all directions so she felt it was okay and less need to be assertive.
Her eyesight also failed ... and I think I didn't realise early enough when the problems first started. Scent marking meant she knew where she was in a confusing world that kept changing (we were decorating) and it was hers. So it might be worth checking out his eyes.
The big thing I did right was protecting a warm space that she choose as hers. It was my desk and over a heater ... so I was there - so she owned the person - and it was warm, and I removed any of the others that thought about intruding. It reinforced a safe space that she didnt have to defend that belonged to the dominant cat .... (still does two dominant cats later!)
Hope that helps!