It all depends on how tight he is. If he has really tight muscles then it might be too painful to do much, but even a little will help. I find it most beneficial to start at either the top or the bottom of the leg and move up an inch at a time, holding in that position until either the pain eases off (which means that the muscle is relaxing) or the pain gets too intense (in which case there is no point pushing it any further that time). When I get to the end of the leg I stop. That might be 5 minutes or 15 minutes depending on how I feel on the day.
Another method is to move up gradually but to rock the roller around the painful spots rather than hold it static. I find this easier to tolerate than holding in just one position as you get pain then relief in quick bursts.
If it is so sore that he can't do either of the above then doing bigger movements down the length of the muscle will help too.
The roller only works on one small spot at a time, so don't forget to move to the left and right of the muscle too so that you cover the whole area. I often find that it is the sides of the muscles that are worst rather than the main body of the muscle.
Its a case of suck it and see. He will soon work out where the painful bits are that need working on the most