Each child is different, which is one reason there are so many ways to help them. What will help your child depends on what they need and how their physical body responds to things. That sounds like a cop-out, but it is true. That's why it is so difficult to find the RIGHT treatment for a specific child.
You are wise to ask what has helped others, but remember that you know your child better than anyone else and especially better than a doctor or therapist who sees them for a few minutes! Listen to the options, think about how your child may respond and PRAY to know what is best for YOUR CHILD.
You will have things that only work for a while and things that don't seem to work at all. Just keep the ideas written down (in a notebook you are keeping about all of this) so you can find them when you need to find a new direction.
Good luck! Just know that many others have gone before you and know the problems/difficulties/fight that you will be going through to help your child be the best they can be.

I use brainchild supplements. They have an autism kit to get u started and walk you through getting started. They are liquid which means they will absorb much easier. My son is in ABA, specifically PRT. And has excelled with this. He gets 16 hours a week and I personally think his therapy help the most. I do not have him on a special diet or other therapies, I prefer to stick with science based evidence. Who knows if that might change in the future. He is only 2 so we have just begun this journey 6 months ago. I

For us the following have helped tremendously...
GFCF diet
cutting down on sugar
Chewable Culturelle
Kirkman Chewable EnZym-Complete/DPP-IV™ w/Isogest®
Vitamin C

PRT is another type of ABA, Pivotal Response Training. I'm thankful for the books I read, that is where I found out how intensive therapy is key at an early age. He has been in therapy for only 4 months and its amazing the progress he has made. The supplements are online. Www.brainchildnutritionals.com. They are expensive but it seems like anything with autism associated is expensive. So I just went for the liquid so I knew he could absorb them. :). I just did a lot if searching for autism supplements and they had everything there i was looking for and they were the only ones with liquid vitamins.

I'm not sure but I think you need a DAN! dr to get B-12 injections....