Let me preface my answer with a few general remarks about child development and stages:
First, not all babies reach the various milestones at exactly the same time. In fact, most milestones have a rather large bracket of what’s “normal”, for crawling numbers are usually between six and twelve months. In short, your baby may be simply on the late side of the Gaussian curve. Only if a baby is missing multiple developmental milestones should you be seriously worried.
Second, not all babies crawl before they start walking. Some start just pulling themselves up and go straight to walking. Others roll, scoot or wriggle around to propel themselves to another place. While a few years (or rather decades) ago pediatricians believed non-crawling to be a warning sign, this is no longer the case - at least not unless other issues are observed, see paragraph above.
In order to learn to crawl, turn, sit and walk, a baby needs strong core muscles and some babies will quite eagerly do something like “baby sit-ups” or “baby leg lifts”. Others less so. But there are ways to encourage them to “exercise” and the nice bit is that at least some will give you a bit of a workout as well.
The most basic thing is “tummy time”, which we usually start early on with small babies. For babies that dislike it (some don't have the patience and complain quickly), a good alternative is to place them on your lap or over one thigh, or even over your shoulder, because then you avoid the face-flat-on-the-floor effect. From PEKIP I learned the following exercise: Put baby high up on your shoulder, so that he's basically balancing just with some support. Turn your back towards a mirror. That's often exciting enough to have them engaging with their own mirror image, training their muscles at the same time. A few minutes a couple of times a day will already acchieve a lot.
For some babies, placing them under one of these baby trapeezes with dangling objects can motivate them a lot - and note than you can add your own items and adjust the height so that they are within babys reach. One of mine enjoyed pulling a thin silk cloth from the trapeeze and over himself. Caveat: Always supervise this kind of play.
When playing with your baby, be as active as possible, e.g. hold them safely by the rump / rib cage and lift them high up, in an "airplane position". Bonus points for making silly noises and funny faces. At nine months, I often held mine with one arm, diapered bottom on my hip, facing away from me. The general idea is to hold them as loosely as possible while still being safe - that means the baby will contribute inadvertently to stabilizing himself. And yes, I bet your baby is getting heavy by now, I did promise you a bit of a workout ;-)
Once your baby has developed his core muscles a bit, there will be way less fussing when you place him in "tummy time" and he will find it easier to try to grab items within or slightly outside his reach. It's your job to find the kind of items (note that I didn't specify "toy") that tempt him. For one of mine it was a "rattle bottle", a plastic water bottle filled with various random small items that made some noise or were interestin to look at - contents could be switched occasionally. The other was a total plush toy fan.
Note that at that age, showing him the motions of crawling, either by yourself or a toy won't work. Mimicking another human's behaviour will come later. Learning works differently at that stage. And crawling comes in stages, as described below.
As a general rule, he needs to learn how to move his body first. Try to give him as much occasion to move as freely as possible (e.g. on a big blanket on the floor instead of sitting in your lap), you can always keep him company if you like and sit on the floor yourself. Either when you are playing with him or simply keeping him company. There are a lot of tasks that can be done sitting on the floor instead of sitting on a table, be creative. You may be surprised how interesting for example a pile of laundry can be that gets folded next to baby - let him “participate”, even if it means that the freshly laundered sock gets chewed and drooled upon and goes right back into the wash instead of the drawer.
And don't worry, all babies go through the stage where they turn in one direction and can't get back. That's perfectly normal (and enervating from an adults' perspective). I would also discourage you from putting him in positions he can't reach by himself (e.g. letting him sit too much). This is based on multiple recommendations we were given by pediatricians and other professionals. Crawling usually starts with a few very weird movements, including pushing up his head and shoulders, pushing the rear high up in the air, generally weaving back and forth, and at some point most babies will end up going backwards when they try to get foreward. Expect frustrated wailing. That's perfectly normal and failure helps them to learn.
Be patient, give him plenty of chances and encouragement to move and if there's no improvement, bring it up with your pediatrician again.
But I had to smile when I read this:
If he can't reach what he wants, he will turn to an adult for help.
I occasionally tell the anecdote that in order to get something, one of my kids learned to walk and the other learned to talk.
Which is both fine per se, if it doesn’t impede their overall development.
