Personally, it sounds to me like your ENT is just playing it safe, which can be good or bad when dealing with special needs children. I have two, so I know how you feel! If I am understanding your post correctly, this is the first time you have had to take him to the ENT? Has he had a history of infections, ear problems, etc? If he hasn't, I would definately wait the 3 mos. Reading throught the rest of the responses, the ones that said push it, their children have had multiple ear infections and problems, so I would agree under those circumstances. Many things can cause fluid behind the ears that can and will resolve itself quickly! I would definately give it a chance to do so.
But, if you are uncomfortable with waiting 3 mos, see if the dr will compromise with like 6 weeks or something like that. If he refuses to compromise, I would seek a second opinion. Doctors that won't listen to parents or compromise with things they are uncomfortable with aren't the ones I care to take my kids to! It is a hard balancing act with special needs children. You have to fight for what you think is best sometimes, but you also have to take the drs advice to heart also! That is why it is KEY to have a dr that listens to your concerns and will compromise.
In the meantime, I would have his speech evaluated. If you have a speech therapist agree with you that the fluid is part of the problem with his speech delay, you can use that to get the dr to move things along quicker and I would be more apt to say go for the tubes! (BTW-my son with cerebral palsy did indeed end up with tubes due to constant ear infections, but it didn't help nor hinder his speech at all...he was still greatly delayed).
I hope this helps allay some of your fears! A lot of drs don't like listening to parents and that is when they don't see me or my children again! But drs also like to have documentation in order to stray from their "norm" routines for dealing with things....
Good luck!!!
H.