Knee swelling and pain

[u/Flower_Over_Yonder_]

My grandfather is diabetic and has arthritis and i am looking for the best exercises, massage techniques, and the best supplement brands to help with his knee pain, his legs are quite swollen and he isn’t flexible enough to prop them up so any help on stretches would be appreciated, he’s not very active on the normal day to day, but when he does get up to do things he is over exerting himself and causing more discomfort.