My 1993 Ford Bronco 351 has a miss in it after it warms up what can I do to fix that

Try cleaning the injectors with sea foam.

what is seafoam

He uses stp fuel injector cleaner in it all the time

No this is injector cleaner you put straight into the injectors not in the tank.

ok thank you I will have him try that

You actually pull a vacuum off the engine and put it in the can and let it suck the can dry. Then shut the engine off for 15 minutes as soon as the can is empty. Re install the vacuum hose on engine. Start it and let it idle for 5 minutes then test drive and run the wheels of it. It will blow white smoke out for 10-15 minutes every time you get on it while driving. This is normal.