Where can I open a TFSA S&P500 tracker for my minor child?

I live abroad but am planning to head back to SA in a year or two. My son has SA citizenship.

Already tried:
X Easy Equities: not possible, because my son needs to "reside" in SA (this is an operational limitation rather than legal requirement).

X Sygnia: not possible, their minor accounts are limited to 3 specific ETFs which doesn't include S&P500.

X Allan Gray: does not offer an S&P500 tracker fund.

I'm looking specifically for an S&P500 ETF since this will be a long term investment (18+ years) so risk can be high but fees must be low.