Its not possible at all. You don't buy the game from David directly, he sells it in different shops. The problem is, that this shops has some restrictions, steam won't work on android, android not at ios and so on.
You can't blame David for that, he's just a victim of the actual market situations. To change that we need help from government because I don't believe the big player will ever cooperate and give us players an all-in-one solution