You mentioned putting Alphie in kennels earlier. If he starts getting the hang of toileting I would not put him in kennels. Murphy has just got back from a weeks stay and had several accidents. Before the kennels he hadn't had an accident in well over two months.
I have to agree with this, Caoimhe was veeeeery slow to housetrain but it wasn't really her fault as I didn't get her until she was over 5 months and she'd spent some time in a kennel up to then and then when she was 10 months and kinda sorta starting to get it she went down to her field trial trainer for the first time and stayed in his kennels for 6 weeks. When she came home it was toileting a go-go in the house and we had to go back to scratch, but then she spent most of the subsequent 3 years going between home and the kennels so she was never entirely reliable when here and I never entirely trusted her not to just go if and when she needed to if I wasn't instantly there to let her out. She only peed indoors though, thankfully we didn't really get poo incidents, and it wasn't all the time, maybe once every few weeks or less which made it all the more frustrating! I do trust her now though - not before time, she was 5 in January! - she has retired from trials and has been at home for the last year and hasn't peed inside for 11 months now (but who's counting
). She still doesn't ask to go out though, none of mine ever do unless they've got a really upset stomach. I'm not saying it's going to take Archie 5 years to housetrain by the way, but I think being kennelled may well set him back given that it's obviously not really his 'thing' as it is.
I'd also add that even with the kennels thing a good part of Caoimhe's issue was laziness on her part; she knew outside=toilet but if I wasn't there to instantly take her out she was quite happy to just pee wherever she happened to be which is something that Ro and Tilly would just never do no matter how desperate they were. She would also sometimes pee in crates and on bedding which apparently dogs don't do! Thankfully we finally seem to have broken the cycle but I admit I still check the floors if I've been out for more than a couple of hours, force of habit
It's nothing to do with how intelligent or generally trainable they are though, she might have ruined several carpets but she still managed to become a FTCh
For the time being I would also consider restricting his space in the garden with a playpen so he has less opportunity to mess around, so to speak. I also think that getting a trainer or behaviourist in would be a good idea though as an objective pair of eyes may spot something that is triggering this behaviour - sometimes when you're living through it yourself it's hard to see the wood for the trees.