The tweet clarifies that he purchased DOGE for his infant son (probably as a joke). On a serious note, Tesla invested $1.5 billion in Bitcoin last month and we can say that Elon has indirectly purchased BTC. So he knows which cryptocurrency to take seriously. He made two cryptocurrency purchases. The first one involved BTC, to increase his wealth and protect it against inflation. And the second one involved a few $$$ worth of DOGE so that his son can play with them.