That's exactly what my boy has been like OP, if i go outside and wander about a bit making encouraging noises he reluctantly came out - but only about four foot's worth outside before dashing back indoors.
We moved here in the first week of October and he is just about getting his confidence back re the great outdoors now, hence my opinion on it being a time thing as well.
Know how you feel though as mine did a complete about turn too - loved to be outside in all weathers & would insist on being outside while i was at work all day (no cat flap here) & then suddenly refused to go out at all.
I was so worried i even posted on here about it at the time.
Trouble is, the more anyone tries to push a cat to do what suits the humans the more they seem to dig their paws in & do their own thing :)
Jake now happily stays in all day and does his rounds outside once i've gone to bed (i leave bedroom window open for him)
Thought he was just sitting on the windowsill guarding against his 'nemesis' coming in but i woke at 1.45 last night & Jake was nowhere to be seen.
Strolled back through the window half an hour later hugely pleased with himself - did say to him 'yer furry little git, there's me worrying about you for no reason - again' :)
Oh, much reduced litter tray usage this weekend as well so presume he's found a nice outdoor spot again now.
Yours will bounce back too, just a bit of a nuisance for your partner right now. Then again, the flippant part of me wants to suggest that a dirty litter is excellent training for the nappies he will be facing in the near future....
I'll get my coat.... :)