When nails are too
long it can affect the whole gait, and yes, put strain on joints and ligaments. The best thing to do is not to ever let the nails get too long in future. The strain should ease up in a few days, and in the meantime, don't put him through any particularly strenuous, or athletic movements. Just gentle exercise, like walking. No agility, or leaping around if it can possibly be avoided. Then things SHOULD heal naturally, and get back to balance.