Thank you for your reply.
There are several causes for inappropriate elimination on the floor, and they can be classified in 2 main groups: medical problems and behavioural problems.
Medical problems would include for example a cystitis (a bladder inflammation), for which the cat is unable to hold the bladder long enough to reach the litter tray. Other conditions, such as metabolic disease which make the cat drinking more (and therefore urinating more) or arthritis (which makes the cat too painful to climb the litter tray) seem very unlikely, given the young age.
Behavioural problem would include mainly anxiety, fear, stress or inappropriate training.
Given the long list of differential diagnosis to potentially explain the clinical signs I would advise, first of all, a complete physical examination performed by a vet, in order to rule out any medical problems.
Once this is done, I would advise to work on the behavioural aspect. Feliway plugs tend to minimise the stress of the animal, as well as plenty of cuddles and play time. Having several litter trays (initially, not indefinitely) both inside and outside will make elimination on the floor less likely.
Also, I cannot highlight enough the importance of cleaning deeply the places where he urinates/defecates outside the litter box, as if even a minimal smell is left on the floor, he will think that’s an appropriate place to toilette.
Usually, with these measures, time and patience we achieve a resolution of the problem.
Does this make sense?