There are a lot of supplements that can help with arthritis, but I think ones that work for a particular dog - or human - can be trial and error. For instance, glucosamine/chondroitin did wonders for me and fast, but I know people it had no effect on. I've used Cosequin for my dogs and seen benefit. I have a girl now with moderately dysplastic hips and she gets Cosequin and Adequan shots. IMO it's the Adequan which makes the biggest difference, but unless you can get the it through a prescription from your vet and give sub-cutaneous shots yourself, it's extremely expensive. Of course my girl is 85 pounds and a smaller dog wouldn't need as large a dose, which would reduce cost.
I've never heard of flax oil having any beneficial effect, but have heard people say fish oil helps. Never seen or experienced any anti-arthritic effect with it.
I'd start by trying a glucosamine/chondroitin supplement for a couple months and see if it helps. If not, try something else. As to helping up stairs, a towel under the back end that enables you to give some support and even lift a little can help. I've got some devices like that but with handles that vets supplied after surgery. Maybe you can get one from your vet. Don't know where you could buy one.
Since a lot of anti-inflammatories and pain relievers can have detrimental effects to liver and kidneys with long-term use, I try to avoid them until nothing else gives relief. At some point relieving pain now matters more than long-term effects with senior dogs.