With my Eldest child: EACH month, I explained to her, how her little brother was changing. Simple. Just explain it to your child.
My Eldest always understood.
Teach your son, that he was like that too. But now he is 3, and STILL learning other things and about himself.
THEN, no... a child does NOT have to always "share" their things. They should also decide, IF they want to share something. It is theirs and some things are just too special etc.
With my Eldest, BEFORE my son was born, I explained to her "Mommy does not expect you to share everything. I understand. Its okay..." and it is. A child, also has to learn that they have boundaries too.
Not ALL things, has to be, shared. My Eldest, would just keep certain things in her room. It was up to her.
I never, made her share everything. Nor do I do that with my youngest. They need to know, that some things can be, just theirs. For whatever reason.
Personally, I don't want to share ALL of my things, with everybody in the house.
It is the concept of respecting others, as well.
a 3.5 year old, is VERY different from a 1.5 year old.
And, the 3's and 4's are a very hard age.
Remember, that even if a child is the "Eldest"... they are still their age and going through their OWN, age related developmental difficulties and growing-pains too.
I know one elder sibling, that would really vent to me about her younger siblings... because, she was always the token default 'example' for all her siblings and she was really stressed out by that. Even if she was a young kid, herself.
Keep expectations, of the Oldest, age appropriate.
And yes, Boys ARE rougher. I have a boy and girl.
Boys...are very different than little girls. They are MUCH more physical and active and have larger motor movements. They just are, per gender, that way.
They are rougher and more physical.
And yes, for an elder child, it CAN be real irritating... to be around a baby all the time and having to defer to the baby all the time all day all the time round the clock. It is aggravating, for the oldest.
HE needs time, to just be himself.
Kids who are 3, do not even have, fully developed emotions yet, either.
NOR do they have the articulation to understand every abstract nuance of emotions nor are they fully socialized yet... and they do not even have... fully developed "coping-skills" either. All of these things are taught. Not even some adults, have coping skills.
He... needs more guidance, per his age now. Than say, a baby. Because, he is on the cusp... of so many things, developmentally and emotionally. They need a "rudder".... so that they don't get negative or resent... the sibling. But need to be, validated too... for their own feelings and who they are.