If there is swelling, pain and the horse is feverish those are signs of joint infection and the vet should be called immediately for an emergency. Joints have very little blood flow and therefore cannot fight infections. Because they are a sterile environment it takes only a few bacteria from a tiny scrape to infect them. The infection can grow quickly, leading to sepsis and death.
Anyways, a horse with arthritis will benefit from moving 24/7 and consistent light riding. You will notice he is stiff moving off after standing for tacking, or being stalled for the night. He may seem "off" at the beginning of a ride and "work out of it". It will come on progressively over months or years, and it will progressively worsen. There are many treatment and prevention options, from the very expensive to the relatively inexpensive, all varying in price. A call to your vet can help you with this.
They say money doesn't buy happiness -- well happiness doesn't buy horses!