Hi there Barbara and thank you for this question.
If your cat is spraying, there can be a few causes for this including not being neutered, a urinary infection, cystitis or bladder stones.
For others, it can be because the tray is not quite big enough for them.
It's worth consulting with a vet if this is a new issue, so they can check for any underlying medical issues.
I'd also consider using a bigger tray and perhaps one with walls/a roof.
We should have 2 trays if he is the only cat, but you say your youngest cat, so we want 1.5 trays per cat. So, for example, 3 cats should have access to 5 trays.
We want these trays spread out and in areas of the home that are quiet and calm with low foot traffic.
In case he is fussy, you may find the signs improve if we clean trays more regularly.