Agree with sandman and will add that many kids with neurological issues and autism are very rule bound and even can be inflexible once they learn something is a rule. So, clear boundaries is important. No touching! No hugging even. Really, I would make that a rule. My son had six years of occupational therapy and here are a couple of things that OT's do: First, they show pictures to a child in groups. First family members. People that are very close to them, mom and dad, etc. Those people they are told they CAN hug. Then they are showed casual friends. THOSE people they can high five. And then they are shown pictures of strangers. Those people they can say hi to. You kind of have to spell it out. So, encourage him to high five your daughter. No hugging, kissing, rubbing, etc! Another thing that OT's do to help with personal space as while some LIKE their own personal space, some kids invade others like crazy. Common problem for kids with neurological issues. So, they can use spacing tactic called either Robot Arms or Bubble. Stick your arms out like a robot and to the side and in front of you, arms length. THAT is how close you can get to someone. Good to make a game of it and pretend to be robots. Bubble is same concept --- that everyone has a space bubble around them. And you can not get into someone's bubble. You can show this with a hula hoop. Put it around someone. They are good. Try to get in their hula hoop and they feel immediately awkward. The hula hoop represents the bubble. I used cue words with my son to remind him, robot arms or bubble and he'd back off of a kid. Good luck
I think rather then getting her more involved in his life (which is a good idea for any mother) - you might focus on a more simple goal. Trying to stop unwanted touching. I would ask her if she has any ideas on how you can help him without scaring him to not be so "touchy" around your daughter. Many kids (well, and especially autistic kids) do not like their personal space to be invaded. So making this not so much about him, but as a way to help your daughter should be less threatening.
Now, one thing that you could suggest is that there is a book aimed at this age group called, "No Means No!: Teaching children about personal boundaries, respect and consent; empowering kids by respecting their choices and their right to say, 'No!'" As one reviewer said, "has taught our child that it's not OK to be forced into hugging, kissing, or touching someone else and that it is OK to tell someone that you don't want to be touched." Buy the book for her to read to her son and for you to read to all of them. More info can be found here.
https://www.amazon.com/No-Means-boundaries-empowering-respecting/dp/1925089223/ref=pd_sbs_14_6?_encoding=UTF8&pd_rd_i=1925089223&pd_rd_r=4AZP4V8S0867FNSJJMKV&pd_rd_w=CTwas&pd_rd_wg=ORBCF&psc=1&refRID=4AZP4V8S0867FNSJJMKV
I also think that since you are dealing with experts on Autism at your health center, that you should get their opinion on what you could do. They would know much more about this then I.
Hope this helps. Best wishes.