Cash is pretty traditional. They are adults in the church and trusting them with money is a big step into adulthood. At 15-17yrs old (the traditional confirmation ages) you don't really need to worry about him using it and cash can be used anywhere. The worst thing that could happen is he saves it for a car or to use while he is away at school.
As for the amount, that is really up to you and how much you value, and celebrate him taking this step in his life, as well as what you can afford.
It is seen as more important than a wedding in my family, as choosing to follow God for eternity is more important than choosing a life mate. Those that are not Catholic are not as excited and treat it more like a birthday. Does that help?
The money is traditionally used to help with our independent travel (at least in our community)and the rest to make us more comfortable in our dorms when we went away to school, so gift cards were not an ideal option in this situation.