Making sure that you are in the same room helps
- use what works - so, touch is good and a pause to allow for him to re-focus before speaking.
Rapid retrieval is the speed with which we can produce known information - one typical test is a sheet with a series of colours and you have to name them as quickly and accurately as possible. There are similar tests using objects, letters, numbers.
Phonological processing is basically seeing the patterns in language - so, spotting words that rhyme, use the same initial letter/sound, being able to break words down into smaller and smaller parts and rebuild them and so, it underpins reading and writing.
Short-term memory difficulties are relatively common but often not picked up in bright kids because they compensate. The reason I bring it up again is it can go hand-in-hand with phonological difficulties or rapid retrieval issues.
I would go back to your Salt friend and ask them where to go next. A lot depends on what you want out of the situation. Ed. psych's do reports and recommendations - so, an overview and you have to get it put into practice. Salts would do the language based testing and could suggest a programme - again, it would be down to you to either fund a Salt or do the work with your DS.
And finally, have you spoken to your GP about the bowel movements? That would be my first port of call.