Given his age, the litter box and grooming issues may be pain/mobility-related. I wouldn’t be surprised if he has arthritis. So simply managing that may help.

As for his weight, I would make sure there isn’t another issue (of course). I don’t think it hurts to bump up the pellets. A bit of alfalfa probably wouldn’t hurt either.

The HRS has a nice article on senior bun care.
http://www.rabbit.org/care/elderbun.html

You could have blood results send onto a different clinic if you are going to go elsewhere. His records from his old vet could be forwarded on also.

This thread may help if your looking into another vet: https://binkybunny.com/FORUM/tabid/54/aft/116849/afnp/278073/Default.aspx

Or if you’re happy to, post your general location and see if our members have any recommendations.

I would look into getting him a sanitary shave.(Either at this clinic or another). It is much easier to keep them clean then and the skin is less likely to get urine burn. The constantly wet fur against the skin leads to that and other skin infections too. With it shaved, you can apply barrier creams to protect the skin further. Or dusting of pure cornstarch.
They could do his ears for him at that time also.

If you are not confident with the clinic you could do the sani-shave at home if you have the right tools. Or try asking a rabbit rescue if any of their volunteers are experienced with doing it. You could give a donation in exchange for their help.
Even some cat/dog groomers do rabbits so thats another avenue to investigate. They’d need to be very careful with his skin.

I agree with LBJ that their may be a mobility issue. Even with simply lifting his tail. Sometimes vet will treat this empirically and put animals on an anti-inflammatory and see what changes are observed. It’s good you are having blood work done because if your rabbit does have to go on daily medications, this will help determine what his body with cope with best.

For weight gain, higher protein would help but if he has kidney issues, higher protein may be problematic, So it would be a good idea to check a vet on this. If protein is alright for him, he could go on a junior pellet or one that is used for show rabbits.
He’s he ever had his molars checked under sedation?
A small portion of rolled oats daily may help him maintain weight…
If he has always struggled with this, Im not certain you will get him to gain much. It might be a matter of just maintaining his current weight.

The problems you describe sounds like they’re age-related. Only the other day I listened to a webbinar on care of the geriatric rabbit. The urine-problem can be caused by osteoarthritis or genral pain or a kidney problem. Osteoarthritis is the most common cause and would explain his self-cleaning issues as well. For osteoarthritis, the vet in the Webbinar, Livia Benato, recommends pain-and inflammation-management with Metacam (an NSAID). Start with a high dose, then taper down til you find the lowest effective dose. Pain relief helps promote gentle exercise – gentle exercise is important. Since self-cleaning is very important for rabbits, he might take it up again with adequate pain management. Metacam is however contra-indicated in rabbits with chronic kidney-failure, so have his kidney-function checked first.

Additional treatments suggested by Livia Benato are glucosamine-supplements, cold laser treatment, physiotherapy, hydrotherapy, acupuncture.

She didn’t address weight-loss issues, probably because obesity is a far more common problem in the elderly rabbit. But it’s not uncommon in rabbits and it’s quite common in elderly horses (horses are very similar to rabbits in many food-related aspects.) Rolled oats or whole oats as Jersey suggests can be a good addition. There are also pellets for senior buns, Supreme Science Selective makes one type for mature (+4 years) rabbits. It’s supposed to help the rabbit keep a good appetite.

That’s good to hear. I hope the pain meds will help solve some of his problems. As for food, I don’t think him preferring the pellets will hurt anything. In fact, they are going to be better for putting on weight than veggies.