How Do You Know if a Child has Should 'Graduate' from Speech Therapy?

I have a four year old that was diagnosed with mild apraxia at 2. He has made great progress and the speech therapist thinks he is done with speech therapy because he can say all the sounds correctly for his age.

However, I'm concerned about the future, I've heard that children don't produce all their sounds correctly until they are about 7, with the 'th' normally being the last sound mastered.

Since he's correctly doing well in speech, will he now learn his last sounds like other children, will be have to go back to speech therapy, should we keep him in therapy now?

TIA