At his age, his hormones are likely a factor. Even babies that potty train can forget those habits with the onset of hormones. Do you plan to neuter him? If so, it should help.
In the meantime, I'd suggest a larger litter box. Skip the pee pads. Make the box more inviting (and tuck it away in a more private area of the enclosure). One idea would be to remove the cage altogether (the one inside the ex-pen) and place a larger litter box in that area. Or some have even removed the top of such a cage and used the bottom for a litter box.
A hay feeder is fine but isn't necessary. You can still put hay in the litter box. I'll link a page on my site that shows a more inviting litter set-up. It's all about what will entice the bun to go in there. As
@Preitler mentioned, potty training is just taking advantage of what they do naturally. Provide an enticing litter box and they will use it.
Here's that link:
https://rabbitsindoors.weebly.com/litter-training.html