Do you have a written agreement with the co tenant or was he on the tenancy agreement with you?
If you both have entered into the tenancy you are both jointly and severally liable. I am sure you are aware that you are now solely liable for the rent and bills.
Any potential claim against the co tenant will need to be based upon the agreement you both had and being able to establish it to the court. in the absence of a written agreement between you both you will need to base this on your course of dealings over the course of the tenancy, I.e evidence of the co tenant's contribution by payment into your account.
It will be your word against the co tenant's. there are no guarantees with court action.