I am also seeing a child who backs and has lateral lisp. It has taken 9 sessions plus parents who work with him in between and he is now making a more consistent /t/ and /d/ with tongue tip to alveolar ridge. There were a few weeks I felt we weren’t going to get there and I also considered a break but with persistence he is getting there. What is happening I think is that his resting tongue position is tongue tip behind lower teeth and his production of t and d (now we have moved it to front of mouth) is done with tongue tip lowered and the blade making contact with the palate. Plus quite an open mouth position. The last couple of weeks i have looked at jaw height- encouraging him to keep his jaw closed more to keep the tongue up. Used a mirror just. Made a big difference almost straight away. Also have just started looking at /i/ and I used the butterfly technique as explanation to get awareness of sides of tongue touching back molars. Planning to add that on to our /t/ and /d/ once I’m sure he understands and hopefully that will help get that consistency and good contact with alveolar ridge as well as setting the scene for future lateral lisp work! Over the weeks we have done:
Front/back concept work
Lots of mirror work and talking about mouth/articulators
We got the t and d forward but not quite right
Worked on awareness of tongue tip to alveolar ridge with cheerios /chocolate spread on alveolar ridge
Jaw height to keep tongue up.
Butterfly technique looking at /i/
He could do his L well so on weeks he was feeling a bit reluctant I would add in more work on listening which I knew he could do and practice of L just to keep it all positive.
Good luck, it’s a tough one! Definitely recommend trying jaw position if you haven’t already.