i worked at a ski/skate shop in high school. I own a few boards, and so do all my friends.
-What makes a good longboard? Trucks? Wheels?
both. for carving, a big wide soft wheel will add tons of stability. nicer trucks are also way more stable. there are some trucks that are good for low speeds, and some for high. I'd just slap some randall r-IIs on there and loosen em up a lot, although I haven't bought a board in 3 years.
-He's about 5'1 and 90 lbs, 13 years old, 14 in December. ..Are there
different sizes of longboards? What size would be good for him? He
can ride a normal skateboard well and do basic tricks, if that makes a
difference.
the bigger the board, the larger the turning radius, and also the more stability. big boards 48+ inches tend to feel more like a surfboard. smaller boards 38 inches and under turn way quicker but are less stable. the common longboard is a 40-45 inch pintail, a good all-around board that can carve well. if he skates, you may want to look into getting him a board with a kicktail, pintails dont have that. sometimes its hard to tell whether a board has a kicktail from pics
-What effect does board flex have on the ride?
the more flexible, the more it will flex at low speeds when carving. stiffer boards will only flex at high speed, but throw back a lot of power. kinda like skis. its all personal preference. loaded style boards are super flexible, good for cross stepping and pumping. my board is rock hard
-What are some good brands he should look into?
loaded, rayne (best boards I've ridden), kracked skull, landyachtz, dregz, arbor
-Do boards with wheel cutouts make a difference (meaning there's no board over the wheels for deeper turning, I guess)?
I've never gotten wheelbite, but it would suck if it did happen. risers usually solve that problem well enough.
-What is a reasonable price for a longboard?
around 200 bucks for everything. you get what you pay for. my friend bought a complete sector 9 with shit hardware and ate it REALLY bad.
-What other things do you need, like accessories? Don't say helmet, we have those...
different wheels. harder wheels for sliding, softer for carving. don't invest in good bearings because most of the time you're going fast enough anyways, just get reds or something. if he's sliding, some gloves. oh and a fat sack of weed.
it's all preference, but the best boards in my opinion are the landyachtz evo, loaded vanguard, dregz supertanker, arbor pintail. oh and rayne makes the best carving board ever, I forget what its called but it blows EVERYTHING else away. all very different boards but there's no way you won't like any of those, with the exception of the loaded cause some people just dont like a lot of flex, and the rayne cause its unlike any board out there.