Autism is diagnosed when someone has significant difficulties in three areas:
Social communication, social interaction, and repetitive or restrictive behaviour.
Social communication relates to understanding intention, so not interpreting tone of voice, not understanding sarcasm, taking things literally, needing time to process information, etc.
Social interaction relates to building relationships and understanding the 'rules' of interaction, so behaving 'strangely', finding friendships difficult to form or maintain, not reading cues from other people, etc.
Restrictive/repetitive behaviours can be repeated non purposeful activity (e.g. rocking/flapping/humming) or routines that serve to calm someone, rather than being necessary. For example, DD2 will put things in her bag so that it 'feels right' - it doesn't matter that she doesn't need the things in her bag. DD1 will say stock phrases to calm herself, such as 'not now, later'.
Being in a world of his own could be being slightly inattentive, or it could be a feature of ASD. Can you draw him back to you? What is he doing when he is in his own world?
The lack of inhibition, I think, is quite normal. DD3 (NT) had no inhibition until about 6 months ago and she's 12.
The mucky face can be a sensory issue or fine motor issue.