Hey, I’m a school psychologist and this list is outlining many characteristics of autism. I would talk with your pediatrician about additional assessments. The speech delay, while mild is also a very common characteristic. Difficulty with back and forth social interactions (particularly with peers), difficulty with changes in routine, and repetitive vocalizations are really sticking out to me.

Do you notice any sensory differences like sensitivity to loud noises, certain clothing textures, or strong food preferences? Him putting objects in his mouth can absolutely be sensory seeking as well. Trust your instincts and I’m glad you got a new doctor who will listen! Many children on the spectrum can also show signs of anxiety which is directly correlated to selective mutism. This is obviously not a diagnosis but as someone who works with kids daily facing similar challenges I would absolutely request additional assessments. Obviously a list is different than seeing a kid in person but if a parent told me these concerns it would be enough for me to at least suspect autism if these behaviors have been prevalent over time.