@Rhiannon2 Yes you can, but depending on how and where you got the device you want to unlock you will have to follow EEs rules on how to unlock that device. Please have a read of THIS.
If your son is on EE you do not need to unlock the device at all.
Did you buy this device yourself new direct from EE or is it 2nd-hand? The process & costs are different.
Welcome to the community. 🙂
Did you get a chance to take a look at the page that @Chris_B referred to?
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