So pick out and hold the hoof over a pail of water and brush with Dawn Dish soap, until his hoof is clean enough to eat off of. That's half the battle right there. Use toothbrushes to get into tight places, right down to the bottom of cracks and clean out any overhangs. Thrush likes to hide. Go after it. If you don't make it entirely gone, it will come back to haunt you.
If it is arthritis, then the farrier needs to come and balance the trim, which is very important to relieving the stress on the bones and stop feeding the arthritis pain. Specifically, a bevel applied to the toe and especially if he's flared forward. Especially if he has white line disease, where the attachment of the wall has been compromised. If the white line was dug out previously by the farrier and a hole has been left, keep that clean too. Thrush will take any advantage into the hoof it can get. This is your homework.
I'd call the vet primarily and speak to him about diet and supplements he may need and if the farrier hasn't been there in 3-4 weeks, then call him in as well and speak to him about the bevel/thrush, balancing the hoof and him coming back often enough to stay on top of the trim and work on getting rid of the mentioned pathology he has.
Between the two of them, you should get an answer and know the right things to do. Go after the thrush with a vengeance anyway. Nothing can build and get stronger on the hoof if thrush is eating it away at the same time.