I guess you have already worked on his sound discrimination , and that he is 100% accurate discriminating t/d in all word positions.
So here are a few more ideas to raise awareness of articulation position:
Licking chocolate sauce off from the alveolar ridge, showing him where to touch his tongue.
Saying tut tut with ingressive airstream (a “bunny sound”)
Pushing the tongue against the back of his front teeth to get the feeling of how to move the front of his tongue.
Chaining front sounds: L, T, L, T, L, T, whilst reminding him to keep his tongue at the front
There is also a more complex way of achieving t, which the Cleft Palate Specialists taught me, where you stick the tongue out of the mouth and tell the child to say the p sound at the same time (with the tongue sticking out) , then you gradually get them to draw the tongue back inside the mouth , whilst still making the “p” sound repeatedly (opening and closing the mouth popping the lips against the tongue whilst tongue is sticking out). Bring the tongue slowly backwards very gradually until it you make a slight change to the movement, in that the teeth are now making contact with the tongue, rather than the lips. I was told that it is important not to refer to the t sound with this technique, so the child does not revert to their stored representation for the t sound. Instead, you can say “we are going to make a funny p sound with our tongue sticking out, then bring the tongue slowly into the mouth and tickle our tongue with our teeth”, or something like that. Am happy to demonstrate this over a video call if you would like.
Hope that helps!