He needs to up the sets and lower the reps per set. He may be doing too little volume, or too much for the legs overall. As a start here's what I'd do:
at the beginning of the workout, and work out legs 2-3 times per week:
10 sets, 3 reps, with a 5rep max weight. 2-1-x tempo.
do this for squats and deadlifts. That's all you need. Don't do the cardio unless it's HIIT training. Anything else will make his legs skinnier.