I recommend an Indian acrylic or ebonite pen. Acrylics look nice, which is always nice in a gift, and ebonite feels nice in the hand. You can get a Noodler's Konrad (which is a piston pen) in acrylic or ebonite for $40, or an FPR Himalaya (which I think has a converter but does not take cartridges, although it can be eyedropper-ed) in acrylic or ebonite for under $30. Both have (or can be had with) flexible steel nibs, the possible interest/novelty of which I think adds to the value as a gift. I've heard that you can ask FPR's Kevin Thiemann (who is on these forums as kethiemann) to make sure your pen is tuned properly before it's shipped.
Alternately, you could get him a fancy ink, like a scented or shimmering or sheening ink. Look up the Goulet video on sheening inks if you want some suggestions. Oh, and Platinum has a line of mixable inks, which might be interesting if you get him a few colours to play around with. They are a bit pricey though; I think it's something like $20 a bottle (there's also a "mixing kit" which comes with some containers and a medium or something).
PS: the standard Konrad is actually made from in resin and sells for half the price, but I don't recommend it because the resin used smells (some people like the smell, some don't) and suffers from drying issues.
Edited by SoulSamurai, 16 January 2018 - 10:33.