Is there something my Fh’s son could do during the ceremony?

You could have him come up during your vows and do a short vow with him! Here's an example I love:

1. Invite Child (or Children) to stand with their parent
Officiant: Let it be known that [Groom and Bride] do not enter this commitment alone. [Groom] brings with him an amazing and vibrant 13 year old son… [Child's First Name], will you please come forward and stand between your Father and [Partner]. Over the years, the three of you have gotten to know each other; sharing meals, hanging out, and just spending time together. You have successfully melded into a modern-day family.Today [Child's Name], your support to this marriage is clear. [Bride] joins this family circle as your father's wife, as a friend that you can count on and a woman who loves both of you very much.

2. Introduction of the child/children through the words of parent and partner.
Officiant: I would like to tell all of you some things that [Groom] and [Partner] think all of you should know about this amazing young person…[Child's Full Birth Name]Your Father tells me…
(Three qualities Father recognizes and/or admires about his child)And [Partner] shared with me that…
(Three qualities [Partner] recognizes and/or admires about the child)

3. Commemorative Gift (bestowed on the child/children by the couple. Examples: necklace with birth stone, bracelet, family heirloom.)
Officiant: As a way to commemorate this momentous occasion, your Father and [Partner] have a gift for you, and they would like to present it to you now, is that okay?

[Parents give gift to child; if jewelry, put it on them. Whatever it is, tell the guests]

4. Vows: to Child from Both Adults (Parent / Partner will need to write these.)
Officiant: [Groom], [Bride] and [Child] would you please join hands to form your family circle?

[Groom/Father] repeat after me…
[Child], I want you to know how lucky I feel every day just knowing you. You are the energy that flows through my veins. You inspire me and I can't imagine the man that I would have become without you in my life. Thank you for accepting Bride into our world. Nobody knows more than you how she has been there for both of us. You are an amazing young man and an honorable human being that I am proud to call my son. I love you.

[Bride] repeat after me…
[Child], I have known you since the day you were born. . . You and your father are the people who know me best in this world, the people I eat with every day, the first I see in the morning and the last at night. The two of you are my strength and my world. We are the fearsome threesome, and I consider you to be one of the most important people in my life. I love you.

4.1 Vows: Three Promises Adults and Child
Officiant: [Groom], [Bride] and [Child] would you please join hands to form your family circle? I am going to ask all of you 3 questions. I would like all of you to answer each of them with, "I Promise!"

Do you promise to love, respect and protect each other from this day forward?
All 3: "I Promise!"

Do you promise to always try to be the best person you can be?
All 3: "I Promise!"

Do you promise to accept the responsibility of being a family, and encourage, and support each other in your new life together?
All 3: "I Promise!"

4.2 Vows: Three Promises from the Children
(When both partners have children these are promises the kids make to the Family Unit. The couple usually creates the questions so that they relate to their children. Kids are standing with their parents).

Officiant: [Kid A], [Kid B] and [Kid C], I am going to ask you 3 questions. I would like all of you to answer each of them with, "I Promise!"

Do you promise to be tolerant, respectful and accepting of each other's differences?
KIDS: "I Promise!"

Do you promise to always work out disagreements so that your friendships can grow stronger?
KIDS: "I Promise!"

Do you promise to keep your rooms clean and the dirty dishes out of the basement?"
KIDS: "I Promise!"

5. Family Blessing

Officiant: I ask that your home be a place of happiness for all who enter it, and a place where the old and the young are renewed in each others' company, a place for growing, a place for music and celebration, a place for laughter and goofing off.

And when life seems to be too much or you just had a rough day, may your home always be a place of refuge where every one of you can find the comfort of always knowing that you will be accepted and loved unconditionally.

My son, who is 13 and on the autism spectrum is actually going to be walking my mom down the aisle and escorting her out as well. He said he wanted to be a part of our big day but not be the center of attention. I know you said both of your parents will be walking down the aisle, what about FH parents? Could FH possibly just have his son stand up with you both?

An 11-year-old is old enough to give input on what he would like to do. But I would also defer to the kid's father on this issue. He knows his son best. I don't think there are any "shoulds" here, other than what is best for your future step-son.

As an aside, you can still have bridesmaids, if you want them, even if your future spouse is not having groomsmen.