It is very unlikely that CX will fast track anyone from nothing to a Gold status. CX does not even often comp people unless they flew CX/KA a lot. They may do that for some corporate deals. It may be easier to ask for special consideration for a DMP member.

The biggest problem for OP's son is that, if nothing can be arranged, it will take him at least 210K miles from nothing to Diamond. Perhaps if his son has already flown CX/KA very frequently before he turns 18, it may be possible for MPC to recognise that and perhaps comp him based on his pre-18 flights.