In the past I have used tracing paper, with a clock face drawn on two separate pieces. Clock-face one shows minutes-time, cloc-kface two shows hour-time: the two, super-imposed, show "the" time. It gets across the idea that a clock-face show two separate bits of information, together.
When that's in place, you can show how the hour hand works, and the minute hand, which counts up all the little divisions, one by one, or uses the markings beside the hour signs to count in fives.
Repetition of the hour hand counting only hours and the minute hand counting only minutes (and the hours being "invisible" to the minute hand) helps.
It sounds as though your ds is getting stuck on that, perhaps?
I also have a clock that is divided in half (to the hour; past the hour) and marked with quarter past, half past, etc. And I have given both my children watches that have those markings + the 24 hr clock.
There are loads of learning aid analogue watches around. I do think that nothing beats those being worn a fair old bit.
But then, I'm old, and fancy schmancy apps are a bit beyond me!
I can't help but feel that if he's interested, the penny will drop quite soon, just because his mind is working away at it under its own steam.