Guests in your home, and in your life
Some people enjoy the idea that we choose our parents when we're in some incorporeal waiting room somewhere. Others get a kick out of the idea that they will meet the same souls they "knew before" to work things out. Sometimes those beliefs become justifications for bad parenting or slack friendship, because one can always say "Well this is the way it was fated to be." I'm not interested in helping anyone justify the mistreatment of others, so if the beginning of this paragraph is the way you see your relationship with your child, please skip to the next page.
If you're still here, this is what helps me when I am stuck for how to act: I think of my child as a guest in my home. He didn't really choose to come here; I brought him here myself. When there were siblings, he didn't choose to have them. Even a child who says "I want a sister" rarely knows what he's talking about; by the time you can produce one he's on to wanting a motorbike or something. No child has a nine-month attention span, and by the time a newborn is a playmate, a year or three have passed. Don't ever blame a child for having a sibling regardless of how much he expressed a fantasy wish for a playmate who lived there.
Being new to the world, and you being his host (and partner), any light you can shed on the mysteries of the world, and any clues you can give him on what's likely to happen and what's expected of him would be good for all concerned. Advise him what might happen at a wedding reception, or a birthday party, or at a place he's never been to before. Show him how to eat a new food he hasn't seen. Help put him at ease if he's nervous. Provide him all the coaching and reassurance he wants, and no more than he wants.
Find ways to accommodate his everyday needs. Step stools, low drawers and shelves, a low hook for his coat and hat, a small chair and table, some snacks he can get to without asking—consider those to be requirements rather than luxuries. Be courteous and generous.