In my opinion...It's all in his memory storage/retrieval system. He just can't retrieve the information because his stress level is interfering with his address look up (ambient internal body information...translate that to mean the worst culprit...his stress level...is now at a different level than when he stored all that wonderful information "counting to 50 & 25 words" so he's looking someplace else in his brain. Find a way to lower his stress level consistently. You have to be Sherlock Holmes to figure out what's stressing him this time...allergies, dust, pushy kids, new cleaning products in the bathrooms, teacher stressed so behaving differently, even a slight (to everyone else)change in school routine that has him fried. Also if you can have him do bi-lateral exercise (anything physical that coordinately uses both sides of the body) that will lower his stress level (he's probably too young to use an elliptical trainer but as he get's older have him give it a try), trampolines although risky also calm some spectrum kids...remember it's all trial and error if what you try works keep it if it doesn't move on knowing that finding out what doesn't work is valuable info too!
Good Luck

My son's academic's are all over the map. What he can do today, he cant tomorrow, and can the next day. It all has to do with many factors...says his BSC...external and internal stimuli

There could be a reason to why, maybe a change in his day, different aide, teacher, not feeling well, or even the atmosphere. There can be lots that triggers this, believe me I have been through it on and off, and I know he can do and knows it, it just depends on how he is feeling and what the surrounding is around him at the time. When things are taught in a meaningful way and interesting to his focusing things are very successful and shows his intellegence. Sometimes they just don't want to do it and they show that they can't remember, but sometimes it just processing the right way to get it out. Just remember the brain is set up like a computer that is stored w/alot of data and info. and when its on overload it shuts down, and until you can find that key to unlock it and keep it moving in the right direction, you may run into forwards, backwards and no response..but they still know it, my son is 17 and very intellegent but when he doesn't want to work or do certain tasks, he won't even though he knows it and can do it. Again there is something going on and you have to figure out what maybe the issue, again a simple thing as not feeling well, headache, etc. Just keep supporting and you will get accomplishments.

On building a desire to read in your kids...and I don't know how you feel about comic books BUT I think they are wonderful for a) motivating an autistic kid to chose to read (VISUAL) and b)contextualizing the written word (again VISUAL).

Also try books on tape...find ones that you have both the book on tape AND the printed book THEN have your kid listen to the book on tape AGAIN & AGAIN & AGAIN & AGAIN & AGAIN & AGAIN & AGAIN & AGAIN & AGAIN & AGAIN & AGAIN & then have them read the actual book AGAIN & AGAIN & AGAIN & AGAIN & AGAIN (this is a situation where perseverance can be used in a positive way)then chose a new book and do it all again. Jonathan Mooney, who wrote "Riding the Short Bus" is functionally illiterate (meaning reading the written word for in depth comprehension isn't something he can do) yet after a very bumpy start in public schools BUT with the absolute belief of his mother (sound familiar) found a different route ending up at Landmark College in Vermont (specializes in helping LD students) transferred to Brown University GRADUATED and was honored as a Rhodes Scholar and attended Oxford University Cambridge England. His mom found any and every non-traditional way to reach, engage and assist her LD kid to achieve.

Automaticity is the key (repeating things again and again and again...) because stress & sensory levels mess with memory storage and retrieval which presents as not being able to do something (that they used to be able to do) because they can't find the "how to do it menu" in memory. Their address "cuing" is off set. An example is if you were given a 10 question test. You might get 1 grade if the answer format was "fill in the blank" BUT you might get a different grade if it was multiple choice IF your memory was sketchy BUT when you see a "right" answer you know it's the right answer...that's cuing from a zero start not being able to come up with it even though you know it's somewhere in your memory BUT seeing it as a choice among 4 you KNOW it's right because you were "cued" by the visual of the right answer.

PRACTICE makes permanent!!! And as Malcolm Gladwell states in his book "Outliers" 10,000 hours is what it takes to master something so use your kids ability to perseverate to have them practice something OVER and OVER again! If there's a computer program where they can listen to someone count while counting along OUT LOUD themselves, then move on to simple addition programs and have them do it (you guessed it) AGAIN & AGAIN & AGAIN...it's about really imprinting information and algorithms (how to do things) 10 million times because that's what it takes for them to still be able to retrieve the info from memory when they are in various levels of stress.

Good Luck!

i also find all this to come to mind my 5 year old knew his age and can tell you how old he was he also knew how to potty in the toilet and has totally back tracked we still contuine to struggle with his re training and its hard i dont know what else to do myself colin new how to count to ten wasnt much but knew and then he doesnt know he knows his abc's but how long is that going to be before he doesnt anymore i feel for my lil guy