my cat is 4 yo
has been a bit under the weather recently, has been to the vet and is fine, not on any medication or anything but will take about 8 weeks to get better -but is carrying on his business as usual, eating, drinking 50/50 time inside & out, sleeping on the children's beds 
he has barely used a litter tray since he was a kitten, prefering to do his business outside
very rarely if he is shut in for longer than normal he will use the tray but that's probably less than every 6 months
he's now started using it everyday -and then asking to go outside -grr!
how do I encourage him back outside -he's costing me a fortune in cat litter!