High RPMs, won't shift

SO I'm trying to help my son who is stationed all the way on the west coast
and we're all the way on the east coast. SO his truck's RPMs like to get up
to about 3,000 but she refuses to shift but then suddenly after a minute or 2,
it finally shifts. We just replaced the sensor on the outside of the trans
because he had a code for that. My husband test drove it a few times before
my son had it shipped to him and it was fine here. His fluid is full. I did tell
him to change the filter when he gets a chance to and I'm going to have him
put an additive in it. I don't know if I'm telling him to do the right thing or if
it's something completely different. He doesn't have the money for a garage
to do it. It's bad enough it had to go to one the minute it was unloaded
because a brake line blew in transit.