The EHCP can continue until 25, or 26 in some situations, if necessary. Although LAs often look to cease before then, but you can appeal if they try to.
You should still have annual reviews post 16. That includes the annual review meeting. If more regular meetings e.g. trembly MDTs are in F of the EHCP, they must still happen too.
You probably have an AR meeting at some point this term since the last one was probably in the autumn term of Y11 in order to comply with the phase transfer deadline. If this hasn’t been mentioned, look at when the AR is due and ask about the meeting.
As part of the AR, it is worth looking at the current EHCP, particularly the preparation for adulthood provision, to see what amendments are needed. PfA provision is often overlooked or skimmed by LAs, but there is so much to it.
Post 16, if DC are to move from one post 16 provider to another, the LA must finalise the amended EHCP at least 5 months prior to the move.
You say DS is at an AP. But your threads a few months ago were about appealing for a special post 16 placement and you were successful at Tribunal?