Diffence in toilet trained inside and outside...

When my bunny is indoors he is litter trained and using the litter tray. But when he is outdoors he seems to not go to the toilet at all wether in the litter tray for outdoors or on the grass. It seems like he's holding it all in until he comes indoors and back in his cage where his litter tray is and he jumps straight to it and does his business.

Is this normal?

I've not come across this personally. It's not recommended to bring bunnies in and out of the house in winter due to the rapid temperature change being bad for them. Maybe he doesn't feel so at ease outdoors?

It is normal for some bunnies who will only "go" in one place.