Regular old clear masking tape, probably a good idea to tape some tracing paper together and trace a imprint of his arm front, back etc.. Then enlarge the image to where it fits and make your design then for the stencil paper same thing tape a few sheets together. Tape your design on it and make your stencil, you can lay it all at once taped but your probably better off doing one sheet at a time and put it together like a puzzle.
If you have a print or drawing the correct size then simply join 2 pieces of stencil paper together and carefully make your stencil by securing the drawing to the stencil paper with masking tape or paper clips and using a ball poin pen (Biro) or sharp pencil trace over the picture taking your time. You should end up with a nice clear stencil, then I would cut slits in either side so that it wraps around the arm without creasing. If this doesn't work out try as Jay said and cut the stencil into smaller pieces (maybe 2 or 3 pieces) and apply from top to bottom making sure that each peice lines up. If you leave the top piece on you can follow the edge of the stencil paper with the next peice. I would make a few stencils and practice applying it on somebody first so that you know what works best before doing the actual tattoo. You could possibly freehand any small bits if necessary.
Draw each section out on paper and stencil each section. And do not attempt to finish the tattoo in one sitting. stencil a quarter of the stencil design and then just tattoo that part so you wont have to worry about wiping off the other stencil. I usually free hand my poly tribals and use "spray stuff" on it to keep it locked in. have fun