There are a few factors to take in consideration and so I'll list what you can do to make sure this is a good gift.
1. Does he already have a few? If so, then you need to ask about them or at least sneak in and see if you can figure out which ones he has and doesn't have. There is a good checklist you can use if you wish to go through and mark down what he has already.
2. Can you afford the more expensive amiibo that are rare? They can be very pricey, and if you're not sure about what the market is like, you may end up buying one that's really expensive just to see it go down in price during a second release of that character.
3. Will he be able to utilize them? If he has an older 3DS, 2DS or 3DSXL, then you will probably want to hold off just a little longer. In a month or so the NFC reader will be out so that amiibo can be used in games. If he has a "New 3DS" version that's the latest of their handhelds, it has a reader built in. If he has a Wii U, then it can also read them right now.
1 - https://www.pinterest.com/pin/195132596331491603/
2 - http://www.amazon.com/Nintendo-Reader-American-Release-Window/dp/B00VKQGG06