I have the TLS 10' #7 and it is ok. a little underlined maybe but it will throw a 200gr tip a mile! I sent back the TLS 10' #6. It is one of the worst rods I have ever cast. Amazingly bad. I get physically ill just remembering it.
Just like with his shoes, your son is growing fast, and will pick up casting quickly. His ability will be quantum leaps further by fall. I would stick with his 9' #7 if he is comfortable with it. In the fall, reasses his needs.
One cast can change your day...maybe your life.