I don't know if this works absolutely but it seems like it's a hopeful approach - we talk about the baby as her baby brother, and ask what we should do for baby brother, etc. We let her do stuff like buy him a stuffed toy, an outfit, etc. Whenever we see smaller children or babies, we emphasize how we will have to be gentle like this for baby brother. I wasn't sure it meant much to her until just yesterday, she was very careful around a smaller toddler at the museum and afterwards, she pointed out that she was very gentle like she was going to be with baby brother! I know she's 18 months older than your son so I wouldn't necessarily expect him to absorb it, or articulate a response like that but it was my sister who suggested the approach.
Thanks it just nevous because my son is very ruff and very jelouis he doesnt even let my b.f. Snuggle with me :)
My sister has 2 children at a very similar age gap. It was hard at first, but her older one was old enough to understand that the younger one was more fragile and needed more help - not that she didn't have emotional outbursts, or could be trusted not to be too rough, but rationally, she understood. They get up to all sorts of shenanigans together now (he is near 2)!