symptoms like swollen and bruised ankle can be due to ankle
sprain ( he might have twisted ), hairline crack at ankle bone again with twist,
soft tissue sprain , combination of all.
For treatment he has to consult a ortho or physio and with physical evaluation (here they may ask for x ray) and other tests. Based on report for minor to moderate issue the conservative management can be done with
physiotherapy treatment and use of crep bandage to stabilise the ankle as well also use hot and cold water by keeping your leg in it in such a way that water can touch all around.
Use proper foot wear and for time being if possible take rest and follow the guideline given by physio or ortho doctor.
Now if there is major damage at ankle level then here doctor may ask to keep a pop plaster or any surgical correction (for complete ligamental
injury) or
steroid injestion followed by again physiotherapy treatment.
Here I have given general guideline on possible causes and treatment for ankle pain I suggest to follow step by step and I am sure this will be helpful for you.
Take care.