It depends how much you want to spend on your nephew Stephen - you tend to get what you pay for in a pencil kit in terms of quality.
There are a few matching pen/pencil sets out there, a couple of which have been mentioned. The Beaufort pencil insert that Paul is referring to is the same insert that comes with the Mistral pencil, and it will serve in a lot of parker refill type ballpoint kits as a conversion. You need to make sure that the tip of it doesn't protrude too much through the nib section of the kit, (generally speaking, the only way you will find that out is by trying it) and if it's OK, careful trimming of the black stalk at the end of the insert will allow it to convert a pen kit to a pencil. There is a choice of either 0.7mm or 0.5mm leads.
Pen kit spares
Another alternative would be a Mistral pen and a Mistral pencil, but you might not want to pay that much.
Beaufort Mistral high end pen kits.