HELLO, TONSILLITIS DOESN'T CAUSES HEAVY NESS OF TOUNGE.   HAVE YOU CONSULT ENT SPECIALIST FOR TOUNGE TIE( MEANS TOUNGE IS ATTACHED TO BASE OF ORAL CAVITY,)? IF IT IS THEIR SOLUTION IS VERY SIMPLE, THE ATTACHMENT IS CUT UNDER LOCAL ANESTHESIA AND YOUR SON CAN  SPEAK PROPERLY. OR YOU CAN GO FOR SPEECH THERAPY WHICH IS GIVEN IN  MULTIPLE SESSIONS. SLOWLY SPEAK BECOMES NORMAL. IT'S THE PROPER TIME FOR THIS THERAPY.