How to Support My Son's Nature and Help Him Be More Assertive Perhaps

There is something to be taught here. Role playing at home so he feels secure enough to ask for what he wants.
Example: I was just out for dinner with my son and granddaughter, she is 31/2. She told her mommy "Mom, I need a straw for my water" (they didnt give us straws obviously when they gave us our water), so my DIL said "Savannah go ask that waiter for a straw"... and so she did.
Teaching them to do little things like that for themselves will help their esteem.

Sounds like what you did was great, age appropriate. This is typical behavior for kids his age, I thihk Jo W's advice was perfect. Just watch him and if as he gets older isnt more assertive and standing up for himself then you can work on assertiveness with him. Good luck!

Some kids that appear unassertive are actually pick my battles kind of kids. I would let it be for now and watch him for when he does stand up for himself. If he never does then you may want to teach him to pick battles.

I guess I don't understand the question. He's sharing, but the other kids aren't. I teach my kids that no matter what, you treat other people the way you want to be treated. Even when they continue to be nasty to you.

Leave it alone. He will start grabbing them back soon enough. The only thing you'll teach him is to start a problem over the toy. If he's fine moving on, leave it alone.

Jilly,
My son was always more of an observer type of kid. At 3.5, he was just as you've described.
He was never the O. leaping into an activity. He would take time to watch, observe and decide to join in--or maybe not. He also had his portion of others "not sharing" or "being mean".
He's now 9 and he's assertive. And reflective on what IS and what IS NOT worth taking a stand for. He's very popular, has tons of friends, and with very few exceptions, is the kid who is a "friend to all". He has, on several occasions, stuck up for the kid being bullied, or slighted.
You're doing fine and so is he. Relax.
Be careful what you wish for! Would you rather he be the O. grabbing the toys or smacking other kids in the face with a ball? LOL