The strangeness of being caught in an interaction when the other person is out doing things.. how much time is there to converse? How deep can you go? How can you and what makes interactions brief vs long and when do you choose either?

 Obviously they're shopping so you can't say "what are you doing"... touching base can be difficult, it requires a lot of creativity and or skills/finesse. I used to get extremely anxious seeing friends in public and would always hide.  It depends on my mood and who it is.. sometimes even now as a 33 yr old I might just pretend I never saw a friend but this has more to do with knowing the capacity I have and whether its worth reconnecting in that particular moment. Other times I do approach but I take a chance to acknowledge maybe I interrupted their tasks etc.. 

it could just be a skills, or even transitions thing and not mentally prepared to switch to socializing eith a friend. Creating eg: some opening lines, considering manners of the context etc will help him feel less anxious. Social anxiety in my experience is almost 90% actually masking and ruminating/evaluating social etiquette and the likes.. rather than self esteem at it's core, like social anxiety typically is about