What does Work experience placement mean?
Here's what I understand.

work placement is the concept of being put in a job place according to your professional experience and it can be organized by a company or an institution to help people find jobs but there's something I don't understand when they use the word "experience" does that mean that the organization requires you to have work experience to place you in the correct job according to your abilities? Because of this guy's disabilities I suppose they also evaluate other traits to see if they are capable and fit for the job. Am I wrong?

Your idea of "work placement" is correct.

"Work experience placement" is more implying that he's being placed to gather experience and/or to have the "experience" of a regular life activity, such as working. Having Downs syndrome, finding a job by himself would be extremely difficult. So, they would get familiar with him, his capabilities and limitations, and then check that against businesses that were open to being involved in the "initiative" to see if any available positions were a match (not necessarily experience wise, but their actual mental, physical, emotional abilities). It's meant to give them a richer, more normal life experience while also providing them a means of income.

You will occasionally see the term used for the non-disabled in a usage that essentially translates to "internship."