You are a great mother to have noticed this issue in your child's development. You are going to be his first and strongest advocate in what ever lies ahead for your little one. Aa you go to and hear from doctors about you little boy, keep in mind that you spend more time with your child and you have a great handle on his developmental stages. Share this important information with the specialist in a very concise, non-emotional and written down form when you go to an appointment. When did he first roll over, when did he hold his head up firmly, when did he pass things from one hand to another, when did he pull up on furniture, all these milestones are important to a doctors diagnosis. In other words, you will have only a few minutes , maybe 20, to "Tell your story' about your son to the specialist so have a journal with his develpment in writing to share with his doctor.
I noticed my son had stopped speaking by his 18th month checkup except for two words. He had a vocabulary of over 80 words before that and I told the doctor. He made light of it and said well he is advanced in his motor skills( he was taking apart the docotors pen and putting it back together again in front of us) so he is just lagging in his speech development was his answer for the loss of his vocabulary. I took out his baby book and showed him all the words he had been saying since he was 12 months old and how he had lost the ability to speak but he had not lost the ability to understand. He then, seeing my list and charts, made an appointment that afternoon for a hearing test and my son was almost deaf in both ears. He had 20% hearing in one ear and 5% in the other. We went to a specialist and had tubes put in and his hearing did improved. (He was allergic to all drugs and so he had quite a bit of scar tissue built up that was interferring with his hearing abilities). I tell you this to say--- You know your child and his development best! Take a strong role in your child's development and work with the doctors but if you know they are missing something that you feel strongly about, PUSH FOR YOUR CHILD"S HEALTH! Listen to your gut and hear what it is telling you a;nd share that with your con's doctors.
What ever your child's issue is you will learn to work it into your family and your life. By bringing it to your doctors attention so quickly your child will be getting a diagnosis sooner and help sooner and he will have the issue addressed sooner and have a better outcome. Have faith and strength to help your child through what lies ahead.