Can you send the link to what you reference? I'm not seeing it, but I did a really quick search.
I see another problem (maybe bigger or the main problem) in that the school is limiting the time for the IEP meeting. If more time is needed for him to make use of an interpreter, they should give it to him. Otherwise, they're not actually providing an interpreter. And if he can't "participate meaningfully" without an interpreter and without enough time given to make use of an interpreter, they are violating his rights as a parent under the IDEA. Not to mention discrimination against him if he has a diagnosed disability.
If the school district continues to be difficult in this area, I would suggest requesting a facilitated meeting, if your state offers those. That would ensure the meeting is scheduled for an appropriate amount of time. Make sure to tell the facilitator that the use of an interpreter requires some extra time for the meeting.