Hi, Jo! I'm a long time homeschooler who also has a degree in education. I have some experience with special needs children in the past, but am certainly no expert in that area. My children (I have 8) haven't had those particular challenges --although there have been many other challenges! The Lord seems to mold us by making sure we all have challenges! :)
The first thing I would say is don't stress! Most boys, autistic or not, are "behind" at this age. Fine motor skills like holding pencils, and verbal skills can be difficult for many boys. I have three sons who have all been very different from each other. Two of them were born talking, but the oldest didn't say a word until he was three. (I figured "that's the way boys are" until the other two came along.) Then, when he finally did start talking, no one could understand him! He seemed to invent his own language! But he is not "special needs," he was just "behind" in his verbal skills. He is now 15 and in 9th grade. He is doing excellent in school and is very intelligent, but verbal skills still daunt him somewhat. He doesn't have much confidence in his writing abilities, but he is still learning to be a passable writer because of the great high school curriculum we use. (Great Books Curriculum from Smarr Publishers--but you won't need that for a while!) He has a quick wit and a very sweet personality, but he's rather reserved with most people because "talking" just isn't his gift.
I didn't tell you all that to talk about MY kids, :) I'm just sharing with you to let you know that some of your son's dificulties are obviously going to be due to autism, and some are just because he's an individual, and, frankly, because he's a boy. If you use a curriculum with lots of "hands-on" and not so much paperwork, and just love on him a lot, he will probably do very well. I applaud you for homeschooling him because even "experts" in autism can't possibly love him and care about his future as much as you do. Congratulations for taking on this challenge.
I think if I were in your shoes, I would teach him letters and numerals by using techniques that use large motor skills like writing in sand or in pudding with his fingers. Sidewalk chalk could be fun too. Lots and lots and lots of reading aloud (you reading to him) would also be great for his verbal and eventual reading skills. Perhaps you could keep reading "sessions" down to just a few minutes at a time a few times a day at first since he has trouble sitting still. Always stop BEFORE he gets fidgety! Eventually, hopefully, he'll be begging you to keep reading. If he can handle five minutes well, try 6 the next time, 7 the next time, etc. He also might enjoy "scribbling" or drawing with markers or crayons as you read--even building with Legos. That would keep his hands and eyes busy while he's listening. (You might take a look at Sonlight Curriculum, The Weaver Curriculum, or just check out lots of library books.)
There is a fantastic math curriculum that teaches ANY kids their math facts without using paper and pencil at first, but is especially good for kids who aren't as "verbal" or "academic." It's called Mathematics Power Learning for Children by Everard Barrett. (I don't know if Home School, Inc. carries it or not.) It's quite inexpensive and very user-friendly. Another good hands-on math curriculum for all ages is Math-U-See, but the disadvantage to that one is that it's pricey. If your budget isn't too tight, he might love that one!
One of the main beauties of homeschooling is that you can do it YOUR SON'S way at YOUR SON'S pace! His day can be totally individualized just to optimize his learning! I think I would get a book like What your Child Needs to Know When, and look at, say, the 5th grade expectations. You can keep some general goals in the back of your mind as kind of a "5-year plan"! There is no doubt in my mind, your son will be especially gifted in some area, and actually hit some of those 5th-grade goals very EARLY, while other goals may be reached rather LATE. That's perfectly okay! :) Hooray for him!
I have a stepson who is 20 and was just recently diagnosed as autistic. I did not raise him, but have known him since he was about 8. He's always been "different" and has presented unique challenges to his Dad and his Mom and siblings. (He was home-schooled too.) I don't know why he wasn't officially diagnosed earlier, but regardless, he has an amazingly brilliant mind in some areas. He could quote statistics about WWII that would blow your mind. He remembers details of battles, numbers of casualties, etc., and shows a remarkable memory for such facts. He is also a computer whiz, and has recently gotten into creating and editing videos. His social skills aren't that great but he was able to get through home school high school, eventually pass the GED, and then complete a computer certification program at the local community college. We have little doubt that he will probably someday outearn his college-degreed relatives.:) Although he's "different" he still has so much to offer.
Your son will probably become (or already is) quite interested in Legos, the Civil War, Medieval History, Dinosaurs...who knows! Something will strike his fancy and you can let him delve into it with all his heart, and tailor his curriculum around it! Someday he could be an expert in his field.
I really hope some of my suggestions have helped. God bless you and him in your efforts! Someday you'll be able to help others who are starting out on the same path that you are.