He actually sold a boatload of TSLA last year [1]. But yes, his wealth is not really expressible in USD.

If you tried to acquire a similar stake in TSLA, the price would shoot up and it would cost you more. If he tried to sell all his shares, it would plummet and he’d get less out.

1: https://www.wsj.com/articles/elon-musks-tesla-share-selling-...