There is no shortage of Tresiba. There is a shortage of single use insulin pens.
This is explained by the charity
Diabetes UK
Reusable pens are far better anyway: they are more robust, include a reminder on the end about when you last took the insulin which is “smart” (I think it can link to LibreLink), they are available in half unit versions, produce far less waste and the cartridges take up less space in your fridge.
My advice is to get reusable NovoPen Echos for you and your son. I recommend ensuring you have a spare pen in case they break. There is no need to change your insulin if it suits you.
If you are using single use pens for your bolus, I recommend changing this too.
I am not sure why single use pens are so popular when they are inferior to the reusable ones. Unless you have a dexterity problem, the cartridges are very easy to insert.