My heart goes out to you because nothing is harder, more frusterating, or misunderstood than the road you have already traveled. There are a lot of unknowns but you seem to have found yourself a compassionate doctor. Good for you!
My 7 year old son was a lot like your son at 2. He qualilfied for 2 hours of speech and 2 hours of OT. At around 3, I took him to an Autism Clinic and he was dianosed with PPd NOS. For years, I worked tirelessly finding him affordable but quality therapy in Speech and OT. I was constantly advocating for him so that the school district would at least miminally meet their obligations. I placed him in preschool so he would have lots of opportunites to socialize.
The results? He attends our local parochial school in 2nd grade and is above grade level in reading and math. He had to test in to get into this school which meant that they found him within a normal range of behaviors and skills.
Does he beat to his own drum? Sure, his speech is still at times hard to decipher but his vocabulary is advance so part of the problem now is that people don't expect him to use the vocablulary he has. His sensory issues are mostly taken care of. He is fussy with certain things like he doesn't like cups with words on them so child restaurant cups are a no go. He has a strong temper and know exactly how he wants things. So, he is not the most popular kid but he is definitely liked and plays with his peers. His gross motor skill are developmentally behind but progress every year.
The most important piece of advice I received was from a Speech Therapist. She said to dispose of all evidence that said he had this dianosis. Why? Once the appropriate therapy is in place, the dianosis can only hold him back. He probably would not be at this great school if I went around at 7 and said he had PDD NOS. Did it help my case when he was younger and I needed more therapy, yes. But do not share this info with everyone because he has the right to lose the label when it does not apply anymore.
And it won't apply at some time in his life. I know that my son will become a productive citizen of this country and will be able to live independently from me. He continues to grow and learn and achieve skills and knowledge I thought he would never acquire. (I honestly thought at some point he would not walk or talk early on.)
So... good luck, stay strong, believe in your child. You are his best advocate.