You can't change the locks - see above. Also, as it is a joint tenancy he has as much right to live there as you do. So if you tried to exclude him by force in any way he could go to Court & get an order to make you let him go on living there.
The only way you can legally exclude him is to go to Court yourself & get an order preventing him from living there. To do this you need legal help & you may not be able to get legal aid for it - depends on income & various other factors. You could try to find a local family law solicitor who does legal aid, & enquire about the possibility (CAB may be able to help with names - or contact Legal Services Commission (google).