it sounds odd to me - if your child (how old is he now?) has sensory issues and is not upset by the motion of the sling or the closeness to you, it sounds to me like that is something that HELPS him, rather than takes away from him. the rocking motion is vestibular movement, which helps babies learn to balance which is vital to being able to sit, crawl, and esp. walk. bouncing does this too, see-saws, and swing. if your child isn't crying in these situations, you having him in a sling and close to you actually helps him.
Now, depending on how old your child is, if your child is old enough to sit (adjusted age), then perhaps having him in a stroller would build his trunk muscles nad give him more room to practice his skills in balancing.
One of the things that helped us (we had a combo problem of low and hi tone areas) with the sitting and trunk control was an excersaucer because it engaged him in different positions. we also bought a bumbo seat (used it for a week) which helped loosen his hip flexors, and toys that really excited him that required him to sit up to use or be on his belly and look up (we had problems with neck control) to use.
maybe your dr. was just suggesting you switch up his physical environment to give him more reason to succeed in different environments?