I agree, I think further tests are needed. With rapid weight loss and urinary issues, Id strongly suspect he has something affecting his kidneys. Testing for E.cunicili would be a good idea. It can cause a range of symptoms. Here is a bit more about it.
http://www.rabbitwelfare.co.uk/pdfs/ECwebpage.pdf
The article is written for British rabbit owners but EC affects rabbits around the world. It also refers to it as a protozoan parasite (like coccidia is) but it is now known as a microsporidian parasite, a type of fungi. I just wanted to highlight that if you happen to do further reading on EC, you will see both terms crop up.
A lot of rabbits test positive to EC but it’s also possible to get false-negatives. So vets will sometimes just treat based on symptoms rather then do the tests.
Because the spores are shed in the urine, cage mates are often treated also.
It might be that Russell has some other type of kidney problem going on (or something else entirely) but EC is something to definitely consider.