When to start booster training with a SN's kiddo?

I'm wondering when I should start C's booster training... my mom isn't going to bother with it, I'm sure (she just recently started wearing a seat belt after listening to me nag her since I was 12). I also don't think she'll bother with another harness seat after the Nautilus is outgrown (he's 90th percentile for height- on the top slots with maybe 1-2 inches of torso growth left). So, I'm wondering if I should get him a dedicated booster (looking at a vivo) for his 5th b-day and just start training him then? The GN seems to fit him in this vehicle OK, but I'd just prefer a dedicated booster, it would be easier to change back and forth the two seats during training. He has high functioning autism (probably will be diagnosed with Aspergers in the near future).

Opinions?

Josef's dx was PDD-NOS as he showed signs of both classical autism and Asperger's. He has since lost his dx yet when he is sick some of his "quirks" come out.

I was NOT ready for him to be in a booster anytime soon when his fifth b-day rolled around, but something changed while we were in WDW. Even DH noticed him being more "responsble" for himself and showing more and more maturity while we were in WDW and when we returned.

Another SN mom needed an EH seat but could not afford a brand new GN. Josef was given the "talk" about moving out of his harness, in Daddy's car only, and was VERY excited about everything. This happened in mid-March and he was AWESOME on his first trip-but then again I gave him the Mommy death glare the whole two minutes he was in it. He actually behaves better in his Monterey then he does his Regent-he the Monterey.

I have another SN mommy friend whose son is HFA, he would NOT leave the chest clip alone as C complained about being "too tight". They took him out of the Summit and placed him in a booster-he has never once moved out of place

I honestly think that even with SN children you have to look at the individual child, just like a "typical" child. Some children are ready to booster train at 4 and 40, some are not. Do you think he is ready to booster train?

DS has PDD-NOS/High Functioning Autism and turned 5 a couple of months ago. We started booster training him right around his birthday. DS has a Nautilus (on the highest harness setting) in my car and for we would move it to DH's car when needed. DS told us that he wanted a "car seat for Daddy's car". When we went to look at seats DS really liked the Turbobooster. We talked about how he would have to sit big and still in the seat. When the TB went on sale, we bought him one.

DS has been doing great in the TB. He rides in it a couple of times a week and he always sits appropriately in it. I still prefer to have him harnessed, especially for long (multi-hour) drives. But I feel that he is able to ride safely in his TB the times he does use it.

I think this is an area where autism actually plays in our favor. DS is very big on rules and doing things right, especially in the car. So that helps a lot.

Yeah- Chris is somewhere on the spectrum between classical autism and aspergers. He meets the DSM criteria for Autistic Disorder, but at the same time he is so high functioning and so social he doesn't fit into the classical autism mold. He has so much language involvement that he doesn't quite fit aspergers, but I do think they will diagnosis him in spite of that.

Anyway, I think he'll either do VERY well with booster or it will be a disaster. I know he takes car safety very seriously (unlike his more typical brother) which should help. Thanks for the replies I feel better about considering starting at 5th birthday instead of the more preferred 6th birthday. Maybe they'll come out with a more affordable seat with an outrageously high top harness before then? One can hope.