If your son does not have behavioral issues in school, then he should not have an aide. Requesting an IEE will not provide you additional information. At this time you know:
1. His diagnosis
2. His academic performance levels (grade equivalents are irrelevant b/c the tests are nationally normed and every state has different curriculum standards)
3. His IQ (low average)
He has had two years in a split program is not making the progress that they anticipated based on his ability level.
I would suggest trying the special class placement, but putting program review into the recommendations at the end of the first marking period. If he's settling in and learning, then you can continue as planned. If it seems overly restrictive, then you can make changes.
Integrated programs for children with significant learning needs are not always successful. When a child is in an integrated setting, the expectation of him is that he can (and will) achieve at the same rate as his typically developing peers. It sounds as though that may not be the case for your son at this time.
You are incorrect in one statement... it is not difficult to go from a special class placement to an integrated placement. It happens all the time.