Any tips to encourage my new boy to use his litter all the time, not just most times

My little boy came to use when he wasn't toilet trained at all. but he copied Bunbuns and within a day was using the litter tray. However the last few days I found that he peed outside the tray a few times. I just saw him go in to do it in the tray, checked it was pee, but then I can find he does it outside again later on. It may not help that Bunbuns decided to use the litter tray as a digging box (started a few days ago, never done it before at all), so when he was in there earlier to do his business she jumped in and started moving stuff around and digging, so he stopped and came out. I really don't mind her doing it, I can change the hay and litter in it, but I wonder if that could be why he pees outside at times.
does anyone have any tips on what I can do? by the way, he does all his poo in the litter tray, just not always all his wee. I would say it's been three to four times since he started using it last week he did it next to it, so I know he understand it, just wonder what I can do to get him to use it all the time. I always clean the area he peed in with vinegar water.

The thing is there has been a few changes recently so this might have upset their habits. I haven't come across anyway of ensuring a rabbit always goes in his litter tray, most will most of the time, but sometimes they forget.

Yeah good point. He is overall very good and I am really proud of him and how well he settled in. he is a right little star