Hi Sarah Jane.
Glad to hear Twix is peeing better now.
Considering Twix is a huge cat dimensionally, I don't think 5.4 is overweight, he's just a big boy.
Did the vet get a wee sample? If so, did he mention anything about crystals or bladder stones? I have three cats with sporadic cystitis so we keep a supply of Hills CD dry cat kibble available. Sly, Tufty and Pursley all eat this and it seems to keep their flare-ups down to a manageable level.
The covenia and metacam jabs (I'm assuming he had the same treatment that my lot get) will help Twix feel better straight away. If his urethra has been inflamed by crystals (that's where the blood comes from) it will be uncomfortable for him to pee for a day or so until it heals properly, but once it has he'll be back to normal.
The thing to watch out for is when he stops peeing but still strains to go. If this happens his tubes are blocked and that would be a life-threatening condition which would need immediate vet attention. Keep an eye out for this, even though he's peeing freely now he can still block up if crystals or stones are present.