Just an observation. My 13yr old boy is holding better watts per kg than me on zwift. Which he is loving, but is inaccurate when we get out on the road. Maybe a calculation error?
Sort of. It’s likely that your son weighs at or below Zwift’s 45kg weight limitation. If he weighs less than you he’ll likely be able to exceed your w/kg with a good amount of effort.
Yeah he weighs 45kg and can hold between 2.5 - 3 w/kg for about an hour where as I’m 82kg and avg around 2.2 w/kg on zwift… but out on the road he struggles to keep up… for now anyway 
Not complaining just an observation. Love zwifting 
This is a common issue but the math explains it all.
2.5 - 3 3/kg for a 45 kg rider equals 112 - 135 watts.
2.2 w/kg for a 82 kg rider equals 180 watts.
More power wins.
This really is true on flats. The only advantage the smaller rider may have on flats is less wind resistance. He needs to stay tucked in and draft because 135 wats will never keep up with 180 watts.
Now, get on a hill, and you will feel the drag and he may have an advantage but the power difference is pretty large.
Thx for the explanation, makes sense when u do the math 
Tim explains this perfectly.
I have a riding buddy who has exactly the same FTP as me if referred to in w/kg. We are both around the 3.8w/kg. However he is a brute of a guy carrying about 20kg more than me (taller/ bigger more muscular frame). When he winds things up IRL on the flat i have to scramble to stay in the big air pocket he creates because i would never keep up when he is winding up over 400w.
I have the last laugh when things turn skywards as i have less weight to combat so expend less energy sitting at FTP than he does.
Swings and roundabouts, as they say.