Readigrass is richer than normal hay, so should be introduced slowly, like you would greens/veggies. It is slightly higher in calcium than normal hay but generally lower than pellets or green leafy veg. It is high in fibre just like hay.
I would reduce the amount of rocket/mint to just one handful a day and see if that works. I wouldn't reduce the pellets, if anything at his age he should be having a few more - adult rabbits need about a tablespoon per kg of their body weight, but baby rabbits need more to grow. If you increase his pellets, make sure you do it slowly.
Also make sure he is drinking plenty of water, as that will help dilute his wee. If he has a bottle, give him a bowl instead and make sure to change the water at least once a day, preferably twice.
If you give him fresh hay more often he may eat more (minimum twice a day for a reluctant hay eater). Getting him into the habit of eating lots of hay from a baby will help prevent health problems later on.
I would try other hay brands to see if he eats more of those. The best hay that Pets at Home sell is Alfalfa King Timothy hay. Most people buy hay online as there is a lot more choice of better quality hay. Buying in bulk can also be a lot cheaper. I get my hay from these websites:
www.thehayexperts.co.uk
www.hayforpets.co.uk
www.timothyhay.co.uk
Calcium in the wee occasionally is nothing to worry about, it is natural. It would only cause health problems if it was long-term or if the rabbit was predisposed to things like kidney stones.