Obviously, I'm no expert but two foods I can recommend are Connollys Red Mills Engage Salmon and Rice, and Symply salmon.
The Symply I only used briefly, as an emergency when my food didn't arrive, but given the sudden transition, my sensitive dog coped really well with it.
The Connollys I can't fault in terms of healthy skin and tummy, easy to pick up poos and no wind. However, it did work out quite expensive to feed.
I'm not familiar with your dogs condition, so I can't offer any input on whether or not grain free should be your first choice, but it's definitely worth considering. Sympy is, but the Connollys isn't. Also, I feed my dogs cod liver oil and evening primrose oil capsules which work out very cheap to feed and have really improved my dogs' coats and hopefully will help keep their joints in good condition.
I hope you find a food you are happy with, and good luck managing your dogs condition.