Hello Dylan's Mum,
It is good that you are keeping a log of your son's words and that you are noticing a pattern. You do want to be a "detective" as you try to figure out if your son is making progress or if he needs some deliberate intervention (speech therapy and lots of practice with you.)
Here is the kind of information I look for when I evaluate a young child, as I did just yesterday (26 months). I look for indications of social relatedness, play skills, and language/communication skills. Does your son like to spend time with you playing and interacting, or does he prefer to play alone? Does he look up at your face often when you are speaking to him? Does he play with toys in the way you would expect a young 2-year-old to do? Does he understand what you say to him? Does he use gestures to communicate with you since he isn't using many words? Does he initiate communication with you very frequently?
I look intently at the child's reactions when asked to say a word. The child yesterday showed definite signs of distress when asked to say words--she would look away, play with her hair, reach toward some other item to "get away" from the request. At home, she falls to the floor crying after they try to get her to say a word several times for something she wants. They have interpreted this as defiance. I interpreted the behavior as avoidance of something that is currently too hard to do (since she readily imitated any action that did not require speech.)
How clear or unclear are your son's word attempts? Significant speech production (articulation) difficulty can inhibit a child's growth of expressive vocabulary. You and your SLP should discuss this possibility.
It is not usual for a child to use words intensively for a week and then not use them again. I have no explanation for what you describing. Word loss or regression is a feature of autism, but it does not, in and of itself diagnose autism. Has your son had a full evaluation to identify all of his strengths and needs? If not, that would be a wise next step. Do you have a developmental pediatrician, clinical psychologist, or other professional who can evaluate your son's overall development? Hopefully that person could help answer the question why your son "loses" words and does not use many words at any given time.
Best wishes to you and your son!
Mary Lou
Mary Lou B. Johnson, M.S.,CCC-SLP
http://www.helpyourchildspeak.com