Hi, I am a speech therapist who used to do ABA and I entirely agree with Krista. ABA is a great tool and has it's place, but it is limited. I did it with a child who was rather severe and it was the only thing that helped control his behaviors. In his case, it was a really good match - but most kids need more at some point. He may benefit from ABA in the beginning, and may need to transition to something else that keeps him more well rounded as he learns skill sets. But discrete trials along with other intervention techniques may work well for your son also.
Talk to the preschool and see what they can offer. ABA is a technique that you need to be certified to do. It is a method of using discrete trials (which means: do step 1 with cues, then without cues until successful. Add step 2, do steps 1 & 2 with cues, then without cues until successful. Add step 3..... so on a so forth for each lesson). Discrete trials can be done without ABA (but you do want to make sure it is a successful approach). I am an advocate for ABA, but I do feel that it only has its place like everything else - if started with ABA - he'll likely need to transition into other approaches to continue to expand his knowledge.
I see you are expecting #4 in August - congrats! But, I know how overwhelming this can be too! Since he's been getting EI - talk to the SLP also. He/she will likely be able to make some recommendations based on what is seen during therapy. Good luck!