Are the connector barrel and pins the same size?
I would get your son to contact the previous house awner and ask for them to return it - as OR or Sky could charge for replacement (probably would not, but they could). And also call Sky and explain.
Check polarity too.
You need:
- same polarity (middle and outer pin being positive and negative)
- size which fits securely
- correct voltage (not higher or lower)
- amperage at least the same (higher is ok).
I don't have one to hand to check the adaptor but I'm sure you can get one - may be much easier than trying to source one via ISP.
If it's 5.5mm/2.1mm as per Pheasant's response which I'm fairly sure is correct (since we did test this using what looks like the same ONT) you need a DC 5521connector.
Have a look here:
https://www.thinkbroadband.com/guides/how-to-stay-on...
The amperage is what may cause you problems if you don't get it right. I'd buy an adaptor rather than try to use USB if it's just powering it.
seb