You're not wrong, just looking at it from a different perspective. Try this one. It's the gym, who cares if we get tossed around. Spend enough time on the mat and you'll get caught as well as do some catching. Forget about it and move forward. He needs to see this is a blast and who cares if he has a hard time with a bigger student that has more time on the mat than him. For better or worse, that's kinda how it works. I remember the first time I, as a blue belt, caught a black belt. I was ecstatic... until the next round when I got crushed by another blue belt. The mat doesn't care and it will smash your ego just as fast as it strokes it. Once I let go of that score keeping mentality, rolling became pure joy. I don't care if I win or lose in the gym, there is no tally sheet. I'm there to learn, if a 130 pound blue belt has a slick guard pass and she keeps passing my guard, guess who I'm going to be pestering to show me that guard pass? Ditto for the 230 pound black belt with an inescapable side control. Hey dude, how the hell did you lock me down like that? Once I see THAT light come on in my students eyes I know they've been bit, this is now a big learning experience for them and they will have a much better time during training sessions.
This is why we don't let white belts spar, and we don't let folks that aren't competitors train with the competition guys. A tabby is a cat just like a Lion is but it doesn't belong in a Lion's pride for it's own safety and because it's can't keep up.
Size does make up for skill up to a point. Once your little dude learns to let the bigger opponent do their thing rather than waste energy trying to stop them or take them down life will get better. I fight for the takedown but I know a guy with 50-100 pounds on me with equal or better skill is going to get me eventually unless I can bait them into any of the backstepping throws so I'm prepared to save my real fight for the ground. Escape and take the back is usually my plan. Most experienced big guys won't fall for the bait if you let them back you up because they've been Uchi Mata'd or Drop Seoi Nage'd into the next time zone a few times and know to avoid an opponent that let's them drive them back to easily.
Less skilled guys that are bigger? Crush them. We had a former pro NFL guy come in to train with us because he wanted to try MMA. First second of first round he explodes into me hard enough I though my ribs were going to give, slams me and then tries to grind me into dust. I escape get his back and choke him out. Stand up, reset. He tries that again except this time I realize his entire game is basically going to be about running me over. So I hit an Uchi Mata. Which is the throw I've hit on every single big guy because they think they're going to push everyone around like the always do and this makes them a perfect candidate for this one. Sucks to be them. My guy hit the ground and I could tell right away he was in full panic mode. He had no idea how this happened, had never been on his back like this, and had no idea how to get out... other than bench press me which was perfect. It was like hitting a day one arm bar. He's a good dude and runs a construction company in this area now. He still trains BJJ and catch-wrestling. So I'm not trash talking him just using that as an recent example of this one time at band camp. Your son will eventually do this in the gym against a bigger, stronger, but less skilled new guy.
Everyone I know in this game has at least 5 of those stories. Over the course of almost 24 years of doing this stuff on a regular basis I've seen that scenario play out at least 50 times. If your son stays in the game that long, and I hope he does, he'll see it and do it just as many times. He might even have to do it on the street should he need to protect himself. Nothing changes an unfriendly douchebag giants behavior like getting a face full of sidewalk courtesy of a drop seoi nage after he just called you out in front of a club full of people.
I was going to put together a list of throws I really dig for busting up the unfriendly giants but this blog post already covers most of them;
https://nuclearchainsaw.com/2014/05/...g-judo-throws/
Learn those, practice those, and if you need to use them do so with audacity and abandon. Their weight moving forward + the umphh you add with your speed and weight + gravity = unfriendly giant suddenly wanting to be friendly while he holds the pieces of his head together.