Yes your son can pursue an MS in Electronics or Automobile Engineering. Although it will be considerably more difficult since his bachelor's degree will not have the same topics of study as his masters, foreign Universities generally take into account the student's individual interest and skills rather than his designated undergraduate qualification.

• He completes online or residential courses in the subjects required for an MS in Electronics/Automobile Engineering (these will be available on the individual University websites)
• Can justify his career path change effectively through his SOP
• Has carried out co-curricular or extra-curricular activities in the required field.
• Has achieved high scores in the requisite exams for admission.

If your son can justify to the admissions committee that he can successfully pursue an MS in Electronics of Automobile engineering, then they will consider his application regardless of his BTech specialization.

Hello 

Basically the MTech is an upgradation of the BTech degree. Usually, the MTech field is selected according to the BTech degree. 

The subjects are in sync with each other.

As food technology and electronics or automobile engineering aren't much related in any terms , I don't think it would be advisable for your son to peruse Mtech in those fields.