You have an intelligent cat, who finds the kind of semi-enclosed space cats love and thinks, "Isn't that nice of my people, setting this up for me!"
You don't need to worry about the cat smothering the baby - that old wives' tale has been around since before I was born, and I'm a grandma - but you don't want them cuddling up together yet. So change the cat's mind about the bassinet before your son arrives. Try lining the inside with aluminum foil, or with loops of masking tape fastened so that the sticky side is up. Either or both of those may be distasteful enough for kitty to decide, "Thanks but no thanks."
Once baby arrives, you won't be using aluminum foil or masking tape in the bed, of course, so your cat may still give it a try. So keep both babies and cats under supervision. Sometimes giving the cat a new blanket of its own - liberally sprinkled with catnip - can be a good distraction from the bassinet.
My cats were always curious about my babies, and that was fine, but they had to learn to leave the bassinet and crib alone. (Once the baby grew old enough to be wiggly and cat-grabbing, there was no problem with the crib.) I gave the cats (and dogs) other attention to help them feel secure, and my children and pets always enjoyed one another.
By the way, you're letting somebody else take care of the litter box, right?