An SLP looks at both speech AND language, so although your ds may be speaking at an appropriate developmental level, he may not be using language appropriately. A well-trained SLP will be able to note this.
OT to aid with the SID diagnosis is a must. In the meantime, read everything you can get your hands on. Definitely the Out-of-Sync Child, but I'd look closely at "is this your child?" by Rapp, and "children with starving brains" by Mc Candless. There is also a highly useful SID/DSI yahoo group. It is a wealth of information about therapies, websites, new books, biomedical interventions, etc . We recently moved from Chicago to Philadelphia, and the SID group has been my primary source for information about new doctors, therapists, etc. Good Luck mama!