If your rabbit can't reach to clean his bum, he could have eg arthritis (so problems reaching the area), or maybe a dental issue (making it harder to groom properly). There could also be dietary issues (making the poo stickier). A short course of metacam shouldn't have any unwanted side effects, and you may notice a real difference quite quickly. Most vets don't pick up the signs of pain in a rabbit. As prey animals, rabbits tend to hide their symptoms.
You have sought professional advice - I would take it. You can always ring up to ask for more details, such as the possible source of the pain, and how to manage a mucky bum. Did you get any other advice or a follow up appointment?
He needs a bum check twice daily to make sure he stays clean. Daily grooming with a comb will help to remove the poo / stop it sticking. If that doesn't help, them he may need bum baths (in shallow warm soapy water, or with a wet cloth) and then drying thoroughly to avoid sore skin. This time of year isn't so bad, but it does put him at risk of flystrike - which is life threatening. So it pays to be vigilant and get it sorted now.
Info on the use of Metacam:
Read Richard Saunders' Meloxicam Focus Piece today. Explore Dosage, Uses, Risks and more.
rabbitwelfare.co.uk