Welcome to the forum!
Rabbit urine can be many shades of yellow, orange, red. It depends on what they eat. Orange isn't any cause for concern.
The humping is due to hormones. Hormones can cause any number of behavioral changes. One of those is a change in potty habits.. or forgetting potty habits altogether. Getting him neutered would remove those pesky hormones. Just know that it can take several weeks after the surgery for hormones to fully dissipate.
I'm surprised he hasn't tried to chew and/or ingest the puppy pads. It is quite easy to litter train him to a litter box. You can read about a proper (safe) setup here:
https://rabbitsindoors.weebly.com/litter-training.html
Just bear in mind that hormones can disrupt training.
For training, it would be best to confine him (initially) to a smaller space. An exercise pen could work well. He needs to learn to use just his litter box.
What pellet brand are you using and how much are you feeding?
You mentioned him eating grass. Is he going outside?
Rabbits should have unlimited hay to eat. Of course hay is just dried grass, but they do need a lot. I'll attach a photo showing what a typical daily diet looks like. Note how much hay is needed per day.
The site already linked above has more care info that should be helpful in giving you an overview of litter training, how to free roam (the process, beginning with confining to a pen), proper diet, bonding, bunny proofing, health, etc.
Also, if you notice the greens in the diet picture, just know that greens need to be introduced quite slowly. The website explains how to do that and which greens are considered safe.