I have worked in theatre industry for twenty years and used to go 4x a week for work - sometimes by myself and sometimes with others. I also take my daughter (age 7) a lot.
I've never been asked to swop seats seeing a "grown up" evening show, and it would be an unusual request. However, if I was being offered a better seat I wouldn't have minded swopping.
Last Christmas I took my DD to see a show and we couldn't book together (similar to your situation). TBH I didnt think beforehand of asking anyone to move, just that she'd sit a few seats away. I thought that, at 6 and a half, she was old enough to cope. As it turned out there was a family of four in between us and they seemed approachable, so I asked if they wouldn't mind shifting towards the centre and they were very kind and did that for us.
I agree with other posters - asking people to move in towards the middle is definitely the only way to do it, and you will have to take your chances! My guess is at a kids show audiences might be more amenable to this. But bear in mind there might be a couple of groups between you and your child (a pair and a three for instance) - so you'd need to get both to agree.
If the thought of asking (or a negative response) really worries you, or if you think your child might be anxious sitting alone, then I would take another poster's advice and book seats one behind the other. Sit your son in front and you can give him the occasional reassuring pat on the shoulder from where you are! (Also the view will be better further back)
Chances are he will feel very grown up having his own seat in either case! Hope you get to go...