Your best bet to prevent it just keep on with what you are doing. As long as he is kept moving it will prevent him becoming stiff. If he does develope arthritis (as my mare did when she was a similar age) Hopefully it will only be mild. I now treat my mare by giving her vegetable oil in her feed (this does the same job as cod liver oil, is a lot cheaper and also helps to keep a healthy coat), keep them execised because when they are stood still for too long they tend to get stiff. I have also (over winter) been using stable bandages on her legs to keep them warm, and prevent her becoming stiff in the morning. Hope this helps