I work with a lot of people who stutter. I assume the adult does not have receptive language difficulties. Do you know if there is any dyslexia or dyspraxia or any other difficulty as well? It would be helpful to rule out if this expressive language difficulty is coming from the stutter or maybe difficulty with organising thoughts? I suppose I am wondering if it is suspected he has a true expressive language difficulty which would be there even if the stammer wasn’t?
Some adults that come to me aren’t diagnosed dyslexic or dyspraxic but will tell me about their educational history and if elements were difficult.
Often I find the expressive language of people who stutter can feel disorganised and not very clear. A lot of them come across as having expressive language difficulties. I think there can be some planning and organisational difficulties related to the stammer that impact access to clear expressive language and clear expression or thoughts and ideas.
I have found that these expressive language difficulties resolve once the person has gained skills in planning thoughts and controlling emotional response, as well as developing confidence with communication.
Just thought I would express this, in case these thoughts haven’t been considered.
Good luck and sorry I have not answered your questions about an assessment for expressive language as I can’t think of one.