He won't get anywhere near -6.00DS if he's only -0.25 now. I'd reckon -1.50 at most.
Myopia is partially environmental, sitting in front of a pc really hasn't helped lots of children's visual development and you may even find that that -0.25 goes away now that his life is hopefully less screen based.
The best thing he can do is not use a screen excessively, when he does need to have regular breaks at least every 30 minutes when he's looking at something in the distance. He also needs to have at least an hour outside most days as that too has a protective effect on myopia development.
Unless he's complaining that he can't see, I'd recommend another test in 6 months.