Lukas is correct, its all based on weight... I would go with the slightly larger size (130-ish) as he is still growing.

Chopper may be a better bet as its less aggressive than the custom small and geared towards beginners... Custom small is insanely good and more durable if plans are to pass down to another child eventually.

http://www.backcountry.com/outdoorgear/Burton-Chopper-Snowboard-Youth/BUR2419M.html

Free lift ticket and lesson come FREE with this board, too!

Given that your nephew is 75pounds I probably recommend the 125cm long board. That puts him right in the middle of the recommend weight range(50-95pounds)--I personally like it when I am towards the high weight range. This also still gives him about 20 to grow. If you think he will be growing a lot and won't be buying a new board when he gets to 95ish pounds. Maybe go with the 131(65-120pounds). He will still be within the weight range. Be ideally go with the 125cm. Hope that helps :)

Well he needs a larger board for sure. I would go anything from a 130-137. Considering that he's probably growing like a weed, something closer to a 137 would be ideal, that way it'll fit him longer. As for his feet getting bigger, the waist width (how wide the board is) around this year's Burton Chopper 125cm (they don't make a 126) is 230mm. So I'm guessing that's really close to the waist width of the board your son has now. So you need to find something with a considerably larger width than that. Especially since he'll probably be in size 8 boots before you know it.
The Burton Dom Grom is one possibility. They make a 134 and it has a waist width of 237mm. It is however geared more towards a park board.
The 135 Custom Smalls and 135 Custom V-Rocker Smalls both have a waist width of 234mm. These boards are also both geared more towards park, but could be used all-mountain as well. I'm not sure how much his boots are hanging over, but if you don't think 4mm or 7mm is enough extra width, then I would look into something that is obviously a little wider. You can search the site for other kids boards that backcountry has. They unfortunately only show the waist width for one size of any given particular snowboard, so you have to go to the manufacturer's website (like burton.com) and research that particular board there to find out how wide they are (waist).
You may want to look into getting him a woman's board as well. Many of the woman boards they make today look just as cool and similar to mens boards, and they would offer a little more flex, have a wider waist than junior boards, and be more suitable for someone his size.
Hope this helps! Goodluck!

Traditional camber.

This is really a personal preference question because of all the variables possible. But anything from a 130-145 would be fine. Considering he's "advanced" I would put him in something closer to the 145 than the 130. Especially if he's riding powder and all mountain. If he likes the terrain park, shoot for something more towards the middle, say 135-140. The most important thing is getting him a nice board that he can improve and advance his skills with.