Welcome to Tractor Forum psj12. Did the tractor clutch ever work properly after the son replaced the clutch assembly? Sounds as if the clutch friction disk is stuck to the pressure plate and or fly wheel. This is not an unusual problem especially if the tractor has been sitting for a long time without the clutch pedal being blocked to hold the pressure off of the clutch disk. A stuck clutch assembly would indeed cause these symptoms. Even if the clutch linkage works properly and fully depresses the clutch pressure plate fingers; the gears will grind due to the clutch disk being stuck to the pressure plate and or flywheel.
Sometimes manually prying apart the clutch disk while the clutch pedal is fully depressed or starting the tractor and operating it will dislodge the stuck clutch, especially if it is hooked up to a heavy load. This can be very dangerous so be aware of the issues involved and be sure the tractor is in an open area with everyone clear of the machine and be prepared to shut the engine down.
If you run a search website search using "stuck clutch" I am sure you find a number of posts that cover this issue.
If none of the suggestions to free the clutch work, you may well have to split the tractor worst case, but with a little luck and use the problem may be resolved.
Again, welcome and be sure to let us know how things go.