My four year old often gets errections when trying to use the toilet, then he gets upset and tries to push it down too which makes it even worse and he gets more annoyed it won't go away! We usually just take hold of his hand and sing a song, or even sing "if you are happy and you know it clap your hands" clapping his hands keeps them away from his penis and the errection soon dissapears.
If he is trying to touch it to make it go down, likely he is just as embarrassed as you are. Thankfully I don't have that kind of thing to worry about being the gender I am.
Hmm I wonder what makes it go down with a non-autistic person? Perhaps that's something he needs to know. After all a lot of social behaviors have to be learned. They don't come instinctive, especially not with autistic people.
Ah bless! Don't all men have problems with continually needing to touch themselves??? LOL. And babies frequently get erections when you change their nappy. But, on a more serious note don't stress out about it. As your child already has a diagnosis of autism I presume he already has professionals involved. I know it is annoying when school keep mentioning things to you that you have no control over. I would love to have a magic wand too. I would field it back to the professionals as to what, if anything, can be done at this stage. Re-directing is good. Is there anything obvious in his environment causing it or is he touching himself causing the erection. Again, I know your son is not the first or last to be doing this. Social stories may help if he is able to use this type of thing just to help him understand what not to do in company. But it depends if he is able to understand/take that on board at age 6.