You are not obliged to give a reference. However, you could state something like:
X has worked for me for y, and despite health problems and a lack of experience has been a cheerful presence in the workplace.
S/he has struggled dilligently to try and keep up with the workload, and has an excellent record for punctuality/politeness/tidiness (anything positive here).
If I can help you further, please feel free to telephone me on the above number.
thanks ethel, actually the guy's politeness is about his one asset. He is not a bad lad, just doesn't seem cut out to be in the job he's in.On this alone, I think he deserves some kind of reference. I'll bear in mind what you said and write something up!
This sort of reference encourages potential employers to call you for informal 'off the record' clarification. Hopefully he'll be more suited to a different job.