To add to the above, get a high-sided litter tray or one with a rim to stop him peeing over the edge, as they like to back into a corner.
Eventually you want to be cleaning the litter tray out every day, especially in summer, but to start with do not clean it out so often, and only disinfect it maybe once a week, and after you do this put some of the old litter back in the clean tray. This will ensure he can still smell the wee on his tray and then he will remember where to wee
If he wees in the wrong place, wipe it up with tissue or kitchen towel and put the dirty tissue into the litter tray, so he can smell it there. Then clean the area with white vinegar, it will disinfect it but will also remove the smell of urine so he doesnt go back.
Dont worry about poo at the moment, its the wee you want to get him doing in the tray, most of the poos will follow eventually (often not till after neutering).
Unneutered bucks are the hardest to litter train, but it can be done! You may find that as he gets more hormonal he may lose his litter training, but the key is perseverence! You need to clean up a puddle in the wrong place asap before he gets into a habit.
If you are having problems, try putting hay ONLY in the litter tray, or with a hay rack above it. Bunnies love to eat whilst they toilet, so that should encourage him.
Check with your vet when they will neuter him. Some would do him now, others would like to wait. He is the right age to be done now, as long as he is the right weight, and I would encourage you to do it fairly soon, before he starts getting too hormonal, the reason being that the longer he has the hormonal behaviours the more likely it will transfer to a 'learnt' behaviour and it may not go away when he is neutered. If you get him neutered at or before 6 months it should be fine though. Remember you have to wait around 2 months after he is neutered before you can bond him, so factor this into your plans
Stick around, maybe read a few articles in the Care Reference section, you'll find out a lot you didnt know about bunnies. And if you have any other questions feel free to ask, even if they sound silly!