The question for consideration here is whether or not this is a conduct, or a capability issue and you need to check your contract to see how persistent absenteeism is classified. Also, check what your contract says about taking on other work and whether or not this can be done without your permission. You need to talk to your employee in respect of the overall number of hours per week he is working across both jobs to make sure he is not in breach of the working time regulations, as this will look at his combined hours across both jobs.
You can also consider whether or not you want to challenge the latest absence on the basis that he was not sick. Call him to disciplinary hearing in line with your company procedure. Put together a list of his sickness absences and provide this to him in advance of the hearing so that you can discuss it with him. Listen to what he has to say and decide whether or not you wish to take any disciplinary action at this stage.
Make sure he understands that it is his obligation to attend work fit and able to carry out his duties and if he cannot manage two jobs and meet his obligations to you then he will need to decide which job he is going to give up. Ensure he understands the consequences if this absence continues. If he is having difficulties making ends meet then surely he cannot afford to lose pay due to sickness and cannot afford to be dismissed due to absenteeism and unreliability.