My rabbit refuses to use the litter tray outside the cage — what am I doing wrong?

Hey everyone,
I’m honestly at my wits’ end and I could use some help.

My rabbit is almost six months old, and he does use the litter tray when he’s inside his cage. But the moment he’s outside — whether in the house or in the garden — he poops and pees absolutely everywhere. Corners, random spots, grass, kitchen floor… anywhere except the litter.

He’s not neutered yet, but I thought that by this age he would at least start to understand the concept. I keep him in his cage when I’m not around, and when I let him out in the garden he’s on a long leash so he can wander in the grass. Inside the house it’s even worse: I can’t let him roam freely because he immediately finds a corner and uses it as a toilet.

I feel like I’m doing something wrong, but I can’t figure out what.
Why won’t he use the litter tray outside the cage?
Is this normal for an unneutered rabbit his age?
Any advice would really help.

Thanks in advance!