Sounds like your FIL needs to write down the combo number or better yet get a lock with a key and put it on his truck key ring. If he or you already have a master lock key for something else, you can buy a re-keyable lock that a locksmith can key to the same lock. That is what we use so 1 key will open anything on our property. Got tired of flipping thru a giant ring of keys while standing in the rain...

I lease a tract south of Aliceville that has 8 locks on the gate. 4 are separate landowners on that road, 1 is Alabama power, and 3 different groups that hunt off that road.