There is no way to know how much and when he buys bitcoins apart from him making tweets or public announcements. There are huge trades happening all day so you don't know which one is made by whom and although the price should rise when he makes those purchase but the market is so big that the price doesn't move by much and there are sellers when the price rises so it is sort of compensated and hence impossible to determine if a big purchase was made by anyone.

Saylor doesn't have the kind of impact as Elon would have so I don't think it matters too much either if you somehow know when and how much he buys. I was curious if there is a connection between them both because if I am not mistaken they both follow each other on twitter and both were always positive about BTC until recently when Elon took a different path.