Definitely an extra suitcase at $50 or $60 is worth the extra fee if you can get him to fill it with the full 50 lbs. Of course, filling that 50 lbs will be pricey....
We always come back to Poland at full limit. Actually 50 lbs of fabric is quite a bit, and will give you a bit of sewing for some time. He can have the carry on space.
That's what we do with our kids. We tell them they pack their own carry ons with the clothes that they want for the month or two that we are in the states. I use the rest of the space. Occasionally they do need to give me a few things to put in the big suitcases. But it works out well as then they don't have a lot of excess clothes in the U.S. as we always end up getting more either given to us or finding (usually at thrift stores). Plus, then I can plan what goes into the suitcases that we bring back. it pays to have newer suitcases or strong duffel bags that are light weight, so you aren't using up precious weight with the weight of the luggage itself like some of the older ones suitcases are.