Actually that is a damn good idea use the following wording
'Hello could I speak to Simon please my name is *insert*'
'Im sorry Simon is not here at the moment'
'Ok thats fine could you give him a message please'
'Sure no problem'
'Could you please inform him that the members of the Mini forum thats he owes parts and money too would very much like to speak to him and could he contact us as soon as possible as we are considering legal action to recover the money owed to us'
That should kick the conversation off nicely and any normal parent would surely be very concerned ask more questions and enquire as to what the problem was. Then you could explain the whole story and how you have been waiting 3 months and how he is just ignoring your phone calls, txts and emails etc etc!!!!
Thats to me sounds like a very good approach. I have had to use something similar before, the parents offered to pay me there and then and were very embarrased.