How can I convert a twin bed into to a cutting table?

You could get a bed frame with a headboard and a footboard and set a piece of wood on top to make a table. When he wants to sleep there, take the wood off and slide behind the bed to hide it.

see above ... and use the space between the plywood/paneling and the mattress to store plastic storage bins with your supplies -- or just put the plyboard on top of the bins ... you could also make an 'apron' to drop down from the edge of the plyboard to hide the bins and the bed frame/storage underneath.

Cari has a good idea, except that usually headboards and foot boards are not the same size. I would suggest no headboard or foot board, instead hinge a 4" X 8" piece of plywood onto the wall beside the bed, attach chains that support the board when you lower it to a level position the right height for you to work on. This should be plenty of space to cut out your patterns. Also you could put a pretty fabric (maybe a sheet) on the underneath side of the board so it would look attractive when up. A locking mechanism could be put in place to hold the board securely when it is raised.

Put the bed on the tallest risers you can find (usually about 8-10 inches). Get a large cardboard folding cutting mat (can be found at fabric stores (about $30 without a coupon) or a very large piece of stiff cardboard. Use the cardboard on the bed for a cutting table. A bed on the tallest risers I could find (which were at Bed Bath & Beyond -about $12) will fit file boxes underneath and the top ends up about 33 inches off the floor (depending on how thick your mattress and boxspring are), which puts it right between dining table height and countertop height. If the cardboard isn't stiff enough because you are quilting or something that uses a rotary cutter, get a thick piece of plywood and have it cut into three or four pieces that will fit nicely on the bed then hinge them together. When not in use it would just fold up.

I would suggest that you purchase a trundle daybed or trundle sofa. The trundle drawer is instant storage for the unused bedding, if you didn't want to keep it made up as a bed all the time, or for sewing supplies.

The straight sides of the two examples below should be sturdy enough to support your cutting surface.

I like Flipturn's idea, makes a lot of sense with storage. A piece of plywood from Home Depot or Lowes would serve as a cutting top, they will straight cut to any size you want.