I don’t really want to tell you what you should do (:o), but I would just say that there is nothing wrong with choosing a more obscure saint as one’s patron saint. For some kids, being less mainstream is attractive.
Also, choosing a patron saint for Confirmation does not mean that one cannot later develop close relationships and devotions to other saints. In fact, we should do this.
That said, I know people who have wished they chose a different saint. When they were young, they simply chose a saint because they liked the name. Now that they are older and know much more about many more saints, they would have chosen differently. To minimize this sort of thing, I would simply encourage learning more and more about many different saints and, of course, praying about it.
I can’t tell you what to do (and I’m not sure what I’ll do when the time comes for my children) but as of right now, if it were me, I would encourage the child to really pray about it and to read up on the lives of many saints before making a final decision. I would do my best not to come off as trying to dissuade them from choosing a particular saint. I wouldn’t want them to feel guilt-tripped into choosing a saint simply because they think it’s the one that I want for them. I’d really like them to choose it for themselves. But encouraging prayer and study would be good.
Think of it this way. Any saint is someone that the Church assures us is in heaven. Whoever your son chooses, you can be confident that the person is a very good role model and someone who is in a very good position to intercede for your son. The more obscure the saint is, the less “busy” they’ll be answering other people’s prayers, too. (It doesn’t really work that way as the saints are outside of time, so it doesn’t matter how many people are praying to them, but it’s funny to think about it that way. :))