They tried to phase my oldest son out last year, we refused to sign and he went into a stay put IEP. We honestly just wanted a 504 but we were told (inaccurately and we knew it) that he couldn't have a 504. I work as a SPED teacher and know the laws. They were very upset with us but we didn't budge. This year we switched him to a different school and we approached them for a 504. They said sure but since he was new to the school wanted to wait a month to be sure he adjusted and then we would meet again. They were concerned that if we exited the IEP and started the 504 and he began to backslide it would take too long to get it switched back. In the meantime they placed him in the gifted class and continued the supports according to the IEP. After a month and he was continuing to do well we revisited the 504. I proposed we drop the IEP and see what happened as he really wasn't using any services at all and keeping up with the rigor of the gifted class. Now here is where we got things in writing. They promised and signed that once the IEP was exited if he began to struggle the 504 would be initiated immediately and to take no more than 1 week to put in place. A 504 would stay in place long enough to get him back onto an IEP if needed.
A 504 is best for a child who only needs accommodations, which is how my son is. An IEP is better for a child who needs additional or modified instruction which my son has never needed. So we feel safe going this route but keep in mind we are in a great school that we trust.

I wonder if your son is receiving any modifications or accommodations at this point. If so the argument can be made that the reason he is doing so well is because those things are working. To remove the IEP could cause him to have problems accessing the curriculum.

Autism is a life long journey. As our children grow and change new issues come up. High school is often different in many ways from elementary and middle school. To remove the "protection" of the IEP could leave your son open to unforeseen problems. It seems to me that many schools are reluctant to address the social skills deficits. If behaviors are not a problem and your son is passing they feel he is "good to go". As his parent I am sure you realize this is not the case.

I would say it is important to try to keep his IEP.

If your child is on the autism spectrum, even if he is high functioning, do not let them phase him out of special ed. Not at 12 years old. You are about to hit some of the more difficult years for our kids on the spectrum, not just socially, but academically. His diagnosis will show up more in middle and high school than it may have before, especially socially and with written expression. Try very hard to keep your services and support. If you need to, hire a special education advocate to attend the meetings at school with you. They will know your child's rights and help you keep his programming in place.