Just a couple of ideas but work with him to make sure his knee is ok.
1. Lying on his side, lift the upper leg slowly, to about a 45 - 60 degree angle and hold for about 5 seconds. Then lower the leg slowly. You can then set up a series of reps based on his strength.
2. standing straight up, extend onto his "tippy toes" for 5 seconds then down. ( A calf raise) You can set up a series of reps to strengthen his calves. As he grows stronger he can hold juice cans in his hands or do a single leg calf raise.
Those should be two good ones to start with.