You've got this. You will need - a white sheet, a brown cord (or any colour cord, depends what you can sensibly lay your hands on), maybe a bit if red cloth if you can sacrifice a pillow case or find one in a charity shop. Ideally some safety pins so you can drape the costume on him and pin it in place before you hand see it. He might want to wear white PE shorts and t-shirt underneath if he has them. Some kind of sandals or bare feet.
Google for a Hercules costume DIY picture, and think "long toga with red sash over one shoulder" so you've got a picture to work from. Basically, fold sheet in half and make smallish slit for head. Drape it over him and bunch up the shoulders to look draped. Pin in place, sew afterwards.
If he's wearing shorts and T-shirt underneath, you don't need to sew down the sides, you can just wrap the cord round the waist as a belt. But if you have time, leave big arm holes and then just sew down the two straight edges so you close the sides. It doesn't have to fit, it's meant to be loose and belted together. So you don't need to cut the sheet, just sew the original edges together.
If you're going for the red on top, get a long strip, attach it to one shoulder only, and let it fall down the front of the costume. Few safety pins if you want, belt goes over the whole lot.
Good luck!