My oldest lad is so skinny that when he goes out in a rainstorm he comes back bone dry. As light as a feather, you could knock him over with one. A paper bag would have to be blasted with a water cannon for him to stand even the faintest chance of punching his way out of it.
You get the picture. He's kind and sensitive, which is all good, but he also cries too easily. When he gets knocked down in the playground – accidentally or otherwise - instead of brushing himself off, he bursts into tears.
He's only eight, but already I see boys in his class piling on muscle, thickening their frames, becoming more solid and robust. I read a report the other day that boys as young as nine are now entering puberty: if that's the case, some of my son's classmates will be talking like Brian Blessed soon.
For me, all of this is a worry. How long before his easy tears and slight frame mark him out as a target for bullies, especially when he moves up to secondary school?
The thought of your child being bullied has to be one of parents' biggest fears. It may never happen to my children, but I want them to be able to stick up for themselves if it does – or to stand up for others if they find themselves on the wrong end of a bully's nastiness.
I was bullied when I was his age. It only lasted a few weeks – because my dad made sure of it. He enrolled me and my brothers into a local boxing club, which gave us all the confidence and the skills to stand up to nasty-pieces-of-work.
That's my plan for my boy before he becomes a victim himself. Other parents have signed up their kids for self-defence classes for a very different reason: because they fear their child being kidnapped and they want them to learn how to fight back if a snatcher strikes.
Are we being paranoid? Even if I am, I'm not sure I can help it. I was raised on a council estate in Manchester. My dad was a tough guy; his dad was a heavyweight boxing champion. My three younger brothers and I were brought up with the confidence to look after ourselves.
That's what I want for my son: to instil in him an inner-confidence that will radiate from him with a message to would-be bullies: "Don't pick on me – you might lose!"
Even so, boxing might be a punch too far for my lad, so I've signed him up for kick-boxing lessons at a martial arts centre.
The promotional blurb read: "By starting young, children are equipped with invaluable skills that stay with them forever. From increased respect and self-discipline to improved confidence, fitness and team skills, martial arts such as kickboxing can shape a child's character and help them take life in their stride."
It sounded perfect – but at first, my son wasn't sure.
"I don't want to get kicked in the face by someone," he said.
"Don't worry, son, you can kick them back," I joked, and then explained he would have padded headgear, gloves, shin-pads and footwear.
"And the harder you work, you get different coloured belts to show how good you are. It's brilliant."
"Cool," he replied. Four lessons in, he's taken to kick-boxing better than I could have hoped for.
OK, he's useless at it – uncoordinated and gangly – but he loves it. The ritual of dressing up, the deference he shows to the instructors, the action hero role he seems to be playing in his own mind as he beats the hell out of a punchbag!
But the best bit, for me, is the confidence it has given him. Perhaps too much confidence!
As I went to collect him from school the other day, he told me about something that had happened earlier.
He had tied a scarf around his head, bandana-style and was demonstrating the moves he'd been taught.
When one lad tried to grab the scarf from him, my boy whacked him across the ear. A proper haymaker!
The boy told the head teacher and my son was picked out of his class and made to do the walk of shame to the head's office to be given a stern – and appropriate – telling off.
"Why did you hit him?" I asked my son later.
"Because he was bigger than me and I didn't want him to think I was scared of him," he replied.
Hmmm, I'm not sure that's what these self-defence classes are supposed to be for: hit him before he hits you.
But I also couldn't help but feel reassured. My little, scrawny beanpole of a son wasn't going to become a bully's target, after all.
"OK," I said. "But apologise to the other boy - and don't do it again."