Hi Potsy,

It could be the early signs of Autism, but it could also be part of a toddlers normal development and awareness of things around him. When my daughter was around this age and becoming more aware of adults around her she was very fearful of strangers, and restricted interaction with them, especially men and louder people, this was sometimes extended to family members she had met previously.
Have you discussed this with your health visiter?
Of course being wary of strangers in this ay and age is not always a bad thing.