Thank you for your question. This idea is based on a philosophical principle of the affect of all interactions on the soul. So the repetitive use of a certain name to address the soul can have an affect on it. If a person is addressed with a strong name then that affects their personality. If they are addressed with a beautiful name then that affects their personality and so on. It has been used in some cases to help children with illness and some people have found it to work. This is probably why some are advising you to change your son’s name. It is not a guaranteed solution as there is not really any basis for it in the scriptural sources as long as he already has a good name. Changing his name is up to you and there is no problem in it.
At the same time, there are some other factors that i hope you have already taken into consideration. At this stage it is important to visit a doctor and explain your concerns. There are some supporting treatments that the doctor may refer you to such as speech therapy. For speech it is important that the child is spoken to and read to by as many people in the family as possible.
May Allah grant you success and a beautiful future for all of your children