Reddit reviews Sarge Knives SK-130 Fishing Kit with 6-Inch and 7-1/2-Inch Fillet knives, 4-3/4-Inch Blade Serrated Knife, Sharpener, Cutting Board and Carrying Case

A good fillet knife is a pretty solid gift. He undoubtedly already has one, but with two he can switch knives when one gets dull. I'd recommend the Cutco Fisherman's Solution as it is the best, but unfortunately it's out of your price range at $90.

So some other options follow:

• [Buck Silver Creek Folding Fillet Knife] (http://www.amazon.com/Buck-Silver-Folding-Fillet-4-Inch/dp/B00389TOWK/ref=sr_1_6?s=sporting-goods&ie=UTF8&qid=1417483864&sr=1-6&keywords=fillet+knife) He probably doesn't have a folding fillet knife and a folder is nice to have because it's more handy to carry.
  
  
• [SK-130 Fishing Knife Kit] (http://www.amazon.com/Sarge-Knives-SK-130-Serrated-Sharpener/dp/B003BR17HE/ref=sr_1_8?s=sporting-goods&ie=UTF8&qid=1417483864&sr=1-8&keywords=fillet+knife) Cheap knives but it's a whole set with a carrying case and cutting board.
  
  
• [Buck Silver Creek Large Fixed Blade] (http://www.amazon.com/Buck-0225BLS-Silver-Creek-Stainless/dp/B001DBBE5A/ref=sr_1_15?s=sporting-goods&ie=UTF8&qid=1417483864&sr=1-15&keywords=fillet+knife) It's a big knife, guaranteed forever.
  
  You get the idea, a good fillet knife is a decent gift for fishermen.

For camping/fishing, what about fishing knives, this fish finder, or any yeti product?

For aviation, what about aviation themed whisky glasses, a model aircraft or drone, replica propellor, or a flight simulator.

There are also a lot of cool aviation themed lamps 1, 2, 3, 4