I would take it all and donate/give away for him if you can't use it or don't want it. If he's giving it to you for free its because he doesn't want to deal with it.
Quick way to tell if fabric is 100% cotton vs cotton/poly is to get a bowl of water (for safety) a match and a little square of the fabric. Burn the fabric edge. Dip in water. Feel the burnt edge. If its flakey and ashy its cotton. If it has a ridge or crisp edge its got poly in it. Hope that helps. I've sorted through several boxes of fabric this way when I inherited them from older family members.