He'll have to do it the same way he bought his current stake: slowly, over weeks and months, a little bit every day. A totally standard thing to ask your investment bank to do.
He bought 4.6% of Twitter in the two weeks he went from 5% to 9.6% when he filed. That's not super slow.
He owns about 70 million shares. Before he announced his stake, the stock had a daily trading volume of about 20 million. I agree it's not super slow, but also not super fast. Probably a reasonable speed given the volume.