I think since he has access to the outdoors, he has designated that as his place to pee. Is he in a covered run out there, or in open air? As a House Rabbit Forum we tend to advocate indoors only unless protected, but some of our members let their bunnies out so it’s not a hard and fast rule at all.
In this instance, however, it seems your bunny has a set habit. Bunnies are creatures of habit, so they can be changed if they like a new set up. I am wondering if he can be trained if he doesn’t have access to the garden for a couple of weeks, with indoor litter pan options to choose from.
But if you are worried about his hay consumption along with erratic pee and poo, it might be a good idea to visit the vet in case there’s a health issue.
A vinegar and water spray on areas where he shouldn’t be going, while enticing him with the hay in the litter box, or even treats, can help herd him to the pan also. You can try putting him in his pan when you see him lift his tail, then give him a treat and praise him while he’s in there. Praise him every time he is in the pan at all. Bunnies react well to positive reinforcement, but never to negative. You are making it worth his while to repeat the good behavior. He won’t need a treat every time he goes, just once in a while. That way he will continue to do it in the hope that maybe this time he gets one. But the hay being there constantly is always a good thing. Try a different kind of hay, like Oat or Orchard Grass to see if he eats it more.