I still battle the shower thing. I can get them in, but getting them to use soap and shampoo is another story. I have taken to sending them back to the shower and threatening to bathe them myself if they don't do it right the second time. This usually works, because as teens, they don't want me infringing on their privacy. I have a jillion partly used toothpaste tubes in the bathroom because I just kept trying different ones until I found one they could stand. I also found that a reward system works well for the whole hygiene thing. I use small things and reward them when they have done everything in a day, week or what ever time frame we need to use, without being hassled. It is my understanding from other parents of ASD kids that this is very common.

Our son also hates brushing his teeth. We used to literally have to hold him down and brush. Not fun.

But his school did a project where they has slips of paper with all the steps in brushing the teeth on them, and they had to put them in order and paste them onto a piece of paper. I swear - that night, he came home, put that project on the side of the sink, and brushed his teeth all by himself.

Now he's still not crazy about it, but he's much more willing to do it when we put that list in front of him (which is now taped next to the sink for him. If you engage him in the project of "how do we brush teeth" and give him that list (and Duncan's list had little pictures of each step on the slips of paper) then it might bring him into the project more. :)

It's not in front of me, but as I recall the steps they used were:

1. Put toothpaste on toothbrush.

2. Brush teeth.

3. Spit toothpaste into the sink.

4. Fill cup with water.

5. Rinse mouth.

6. Spit water into the sink.

7. All clean! (And the cutest part is that Duncan smiles and says "All Clean" when he gets done!)

I don't think toothpaste is as important as mouth wash. Dilute it, dip the toothbrush in and do the back teeth first. Then let him do brush all his teeth (even though there isn't anything on the brush) and have him "shine" up the front teeth with more wash for a great smile. Then he would sip, swish and spit with the rest of the wash. I also told my son to let me inspect his breath for smells. "Great job! Now you won't gross anyone out today!" Then we moved up to diluted toothpaste. Ha. Worked for us.
The gagging and vomiting could be from feeling pressured.

I have battled the toothpaste thing with my son for years. I finally talked to his dentist and the solution we came up with is for my son to brush with only water, rinse well and swish mouthwash. Finding a mouthwash he would use was challenging but now that we have that figured out he brushes almost everyday.