Hey Sog, welcome to the forum
firstly let me start by saying it is customary to introduce yourself here, check out the caption quoted between stars above
any way that out the way, down to the question you ask... well in short any midi keyboard will do the task you require and just to confirm it is so your son can input notes using both hands into sibelus, correct? the reason there is so much variation varies from build quality, key action (semi-weighted or hammer action) and features. How much do you want to spend? i would also like to mention that if you are not using an audio interface you may run into latency issues, latency being the time it takes from the key on the midi controller being pressed and for the computer to register it and you hearing the sound so its 2 stages, midi controller-computer-sound. so if there are latency issues the notes input by your son will be late.
so to keep it very simple, any midi controller would suffice as they all do the basic fundamental job. it is simply down to preference on which one and how fancy you want it feature wise. you will more than likely be able to connect via usb in a midi emulation. if you don't have an audio interface you may need to purchase one IF you run into latency like i described earlier