My hubby keeps getting knee pain. there's no swelling or anything like that. would there be any particular exercise for it? thank you

Exercise depends: What would best help is determined by the type of problem that is causing the pain. Pain free arc exercises are generally advised as well as isometric contractions of the quadriceps. But get a diagnosis to get the right treatment/exercise program.

Probably instability: Probably related to joint instability. Exercises to strengthen quadriceps muscles and assuring good foot mechanics generally helps. Correct the instability and you will likely be able to exercise.