John:

Chuck’s advice is dead-on. Have your F.I.L. check his owners manual and it will tell him the CGVWR. You can certainly exceed these weight ratings, but not safely. There are many places to weigh your F.I.L.’s truck to give you the baseline.

It’s OK for the tow vehicle to weigh less than the towed vehicle. Happens with big rigs all the time. I have an ’81 F-100 which weighs about 4,000 lbs. I have used it on occasion to tow our 4,500# trailer. The most important accessories that let us do this are the load-leveling hitch and the sway control system. Without either of them I would never leave the driveway.

Good Luck.

Bruce