Upvote:9
You are experiencing a condition called 'post-submission anxiety', it's common for first-time applicants. They start to brood over their application, surf the net about it, and slowly work up to a full on flipping out. There's naught to be done for post-submission anxiety; the only remedy is to brass it out.
On to the important point your question raises...
The VFS are couriers who have been trained in collecting biometrics. What happens over the course of time is that some VFS employees take to mind that they are visa experts and start doing what you experienced: i.e., assessing people's applications and giving advice. There have been serious and tragic outcomes from this causing unwarranted refusals. The UK government is aware of it and of course is working to stop it.
Based upon what you wrote, you should have ignored the VFS. Assuming that you listed your son as a family member...
There was no need for your son to invite you and write a letter of invitation, and certainly no need to include your son's financial details in your application. His invitation letter is irrelevant and introduces an unneeded layer of complexity to the application.
Next time a VFS employee starts acting like a lawyer, you can tell him to get back to his station. I.e., ignore it.